Measurement of cerebral monoamine oxidase B activity using L-[11C]deprenyl and dynamic positron emission tomography

An update on the use of image-derived input functions for human PET studies: new hopes or old illusions?

BACKGROUND

The need for arterial blood data in quantitative PET research limits the wider usability of this imaging method in clinical research settings. Image-derived input function (IDIF) approaches have been proposed as a cost-effective and non-invasive alternative to gold-standard arterial sampling. However, this approach comes with its own limitations-partial volume effects and radiometabolite correction among the most important-and varying rates of success, and the use of IDIF for brain PET has been particularly troublesome.

MAIN BODY

This paper summarizes the limitations of IDIF methods for quantitative PET imaging and discusses some of the advances that may make IDIF extraction more reliable. The introduction of automated pipelines (both commercial and open-source) for clinical PET scanners is discussed as a way to improve the reliability of IDIF approaches and their utility for quantitative purposes. Survey data gathered from the PET community are then presented to understand whether the field's opinion of the usefulness and validity of IDIF is improving. Finally, as the introduction of next-generation PET scanners with long axial fields of view, ultra-high sensitivity, and improved spatial and temporal resolution, has also brought IDIF methods back into the spotlight, a discussion of the possibilities offered by these state-of-the-art scanners-inclusion of large vessels, less partial volume in small vessels, better description of the full IDIF kinetics, whole-body modeling of radiometabolite production-is included, providing a pathway for future use of IDIF.

CONCLUSION

Improvements in PET scanner technology and software for automated IDIF extraction may allow to solve some of the major limitations associated with IDIF, such as partial volume effects and poor temporal sampling, with the exciting potential for accurate estimation of single kinetic rates. Nevertheless, until individualized radiometabolite correction can be performed effectively, IDIF approaches remain confined at best to a few tracers.

Imaging Neuroinflammation in Neurodegenerative Disorders

Neuroinflammation plays a major role in the etiopathology of neurodegenerative diseases, including Alzheimer and Parkinson diseases, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis. In vivo monitoring of neuroinflammation using PET is critical to understand this process, and data are accumulating in this regard, thus a review is useful. From PubMed, we retrieved publications using any of the available PET tracers to image neuroinflammation in humans as well as selected articles dealing with experimental animal models or the chemistry of currently used or potential radiotracers. We reviewed 280 articles. The most common PET neuroinflammation target, translocator protein (TSPO), has limitations, lacking cellular specificity and the ability to separate neuroprotective from neurotoxic inflammation. However, TSPO PET is useful to define the amount and location of inflammation in the brain of people with neurodegenerative disorders. We describe the characteristics of TSPO and other potential PET neuroinflammation targets and PET tracers available or in development. Despite target and tracer limitations, in recent years there has been a sharp increase in the number of reports of neuroinflammation PET in humans. The most studied has been Alzheimer disease, in which neuroinflammation seems initially neuroprotective and neurotoxic later in the progression of the disease. We describe the findings in all the major neurodegenerative disorders. Neuroinflammation PET is an indispensable tool to understand the process of neurodegeneration, particularly in humans, as well as to validate target engagement in therapeutic clinical trials.

Monoamine Oxidase Inhibition by Plant-Derived β-Carbolines; Implications for the Psychopharmacology of Tobacco and Ayahuasca

The monoamine oxidases (MAOs) are flavin-containing amine oxidoreductases responsible for metabolism of many biogenic amine molecules in the brain and peripheral tissues. Whereas serotonin is the preferred substrate of MAO-A, phenylethylamine is metabolized by MAO-B, and dopamine and tyramine are nearly ambivalent with respect to the two isozymes. β-Carboline alkaloids such as harmine, harman(e), and norharman(e) are MAO inhibitors present in many plant materials, including foodstuffs, medicinal plants, and intoxicants, notably in tobacco (Nicotiana spp.) and in Banisteriopsis caapi, a vine used in the Amazonian ayahuasca brew. The β-carbolines present in B. caapi may have effects on neurogenesis and intrinsic antidepressant properties, in addition to potentiating the bioavailability of the hallucinogen N,N-dimethyltryptamine (DMT), which is often present in admixture plants of ayahuasca such as Psychotria viridis. Tobacco also contains physiologically relevant concentrations of β-carbolines, which potentially contribute to its psychopharmacology. However, in both cases, the threshold of MAO inhibition sufficient to interact with biogenic amine neurotransmission remains to be established. An important class of antidepressant medications provoke a complete and irreversible inhibition of MAO-A/B, and such complete inhibition is almost unattainable with reversible and competitive inhibitors such as β-carbolines. However, the preclinical and clinical observations with synthetic MAO inhibitors present a background for obtaining a better understanding of the polypharmacologies of tobacco and ayahuasca. Furthermore, MAO inhibitors of diverse structures are present in a wide variety of medicinal plants, but their pharmacological relevance in many instances remains to be established.

Tracers progress for positron emission tomography imaging of glial-related disease

Glial cells play an essential part in the neuron system. They can not only serve as structural blocks in the human brain but also participate in many biological processes. Extensive studies have shown that astrocytes and microglia play an important role in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, as well as glioma, epilepsy, ischemic stroke, and infections. Positron emission tomography is a functional imaging technique providing molecular-level information before anatomic changes are visible and has been widely used in many above-mentioned diseases. In this review, we focus on the positron emission tomography tracers used in pathologies related to glial cells, such as glioma, Alzheimer's disease, and neuroinflammation.

Evaluation of monoamine oxidase A and B type enzyme occupancy using non-radiolabelled tracers in rat brain

Monoamine oxidase (MAO) enzymes, type A and B metabolise the amine neurotransmitters of the body. Selective inhibition of either enzyme is an approach for treating neurodegenerative and stress-induced disorders, and inhibition of an enzyme is proportional to the binding of the MAO inhibitor. Conventionally, the binding of test compounds to enzymes is assessed by radiolabelled ligands in ex vivo and in vivo occupancy assays. Regulatory restrictions and turnaround time are the limitations of the methods that use radiolabelled ligands. But the use of non-radiolabelled tracers and sensitive mass spectrometry (LC-MS/MS) based assays accelerated the determination of target occupancy in pre-clinical species. A report on use of non-radiolabelled ligand in in vivo MAO occupancy assay is not available. The objectives of the present study were to optimise non-radiolabelled harmine and deprenyl as selective tracers in MAO-A and MAO-B occupancy assays and evaluate MAO occupancy of test compounds in rat brain. Tracer optimisation resulted in a detectable, stable, and low ratio (<3.0) of tracer concentrations between any two brain tissues. In occupancy assay, tracer was intravenously administered (10 μg/kg, harmine or 60 μg/kg, L-deprenyl) after the treatment with test compound (clorgyline or tranylcypromine or pargyline or phenelzine or thioperamide). Specific brain tissues were isolated at a defined interval and tracer concentrations were quantified using LC-MS/MS method. Pre-treatment with MAO inhibitors resulted in a decrease (maximum, 80-85%) in harmine or an increase (maximum, 85-300%) in L-deprenyl concentrations. But we considered the change in tracer concentration, relative to the vehicle and positive control groups to calculate MAO occupancy. The observed selectivity and ratio of occupancies (ED₅₀) of test compound towards MAO-A and MAO-B are comparable with the results from in vitro radiolabelled ligand-based inhibition assay. The results demonstrated the application of these non-radiolabelled tracers as suitable pre-clinical tools to determine MAO occupancy.

Head-to-Head Comparison of the Two MAO-B Radioligands, 18F-THK5351 and 11C-L-Deprenyl, to Visualize Astrogliosis in Patients With Neurological Disorders

Three patients with neurological disorders (cerebral infarction, progressive multifocal leukoencephalopathy, and multiple sclerosis) underwent F-THK5351 and C-L-deprenyl PET on the same day to visualize lesions undergoing astrogliosis by measuring MAO-B activity. BPND map and SUV image with F-THK5351 as well as Ki map, Ki/K1 map and SUV image with C-L-deprenyl were created. F-THK5351 BPND maps and SUV images clearly identified the lesions undergoing astrogliosis. C-L-deprenyl Ki/K1 maps were close to F-THK5351 images, but very noisy. Ki maps and SUV images were likely affected by the effect of blood flow. Hence, F-THK5351 is superior to C-L-deprenyl for visualizing lesions undergoing astrogliosis.

Exploring occupancy of the histamine H3 receptor by pitolisant in humans using PET

BACKGROUND AND PURPOSE

BF2.649 (pitolisant, Wakix®) is a novel histamine H₃ receptor inverse agonist/antagonist recently approved for the treatment of narcolepsy disorder. The objective of the study was to investigate in vivo occupancy of H₃ receptors by BF2.649 using PET brain imaging with the H₃ receptor antagonist radioligand [¹¹ C]GSK189254.

EXPERIMENTAL APPROACH

Six healthy adult participants were scanned with [¹¹ C]GSK189254. Participants underwent a total of two PET scans on separate days, 3 h after oral administration of placebo or after pitolisant hydrochloride (40 mg). [¹¹ C]GSK189254 regional total distribution volumes were estimated in nine brain regions of interest with the two tissue-compartment model with arterial input function using a common V_ND across the regions. Brain receptor occupancies were calculated with the Lassen plot.

KEY RESULTS

Pitolisant, at the dose administered, provided high (84 ± 7%; mean ± SD) occupancy of H₃ receptors. The drug was well-tolerated, and participants experienced few adverse events.

CONCLUSION AND IMPLICATIONS

The administration of pitolisant (40 mg) produces a high occupancy of H₃ receptors and may be a new tool for the treatment of a variety of CNS disorders that are associated with mechanisms involving H₃ receptors.

Determination of the Input Function at the Entry of the Tissue of Interest and Its Impact on PET Kinetic Modeling Parameters

Quantitative positron emission tomography (PET) imaging is employed with several measurement protocols all relying on the a priori determination of the input function (IF). The standard technique to determine IF is by blood sampling. However, a unique IF determined in a subject for a given PET study, either defined by sampling or in the images, and commonly utilized for all analyzed tissues in that study equally at rest and during interventions, is expected to provoke biases in the rate constants and in tissue blood volume. The determination of a specific IF at the site of the tissue to be analyzed enhances PET accuracy and renders PET imaging less invasive.

A Functional Approach to Deconvolve Dynamic Neuroimaging Data

Positron emission tomography (PET) is an imaging technique which can be used to investigate chemical changes in human biological processes such as cancer development or neurochemical reactions. Most dynamic PET scans are currently analyzed based on the assumption that linear first-order kinetics can be used to adequately describe the system under observation. However, there has recently been strong evidence that this is not the case. To provide an analysis of PET data which is free from this compartmental assumption, we propose a nonparametric deconvolution and analysis model for dynamic PET data based on functional principal component analysis. This yields flexibility in the possible deconvolved functions while still performing well when a linear compartmental model setup is the true data generating mechanism. As the deconvolution needs to be performed on only a relative small number of basis functions rather than voxel by voxel in the entire three-dimensional volume, the methodology is both robust to typical brain imaging noise levels while also being computationally efficient. The new methodology is investigated through simulations in both one-dimensional functions and 2D images and also applied to a neuroimaging study whose goal is the quantification of opioid receptor concentration in the brain.

High-throughput screening and radioligand binding studies reveal monoamine oxidase-B as the primary binding target for d-deprenyl

AIMS

d-deprenyl is a useful positron emission tomography tracer for visualization of inflammatory processes. Studies with [(11)C]-d-deprenyl showed robust uptake in peripheral painful sites of patients with rheumatoid arthritis or chronic whiplash injury. The mechanism of preferential d-deprenyl uptake is not yet known, but the existence of a specific binding site was proposed. Thus, in the present study, we sought to identify the binding site for d-deprenyl and verify the hypothesis about the possibility of monoamine oxidase enzymes as major targets for this molecule.

MAIN METHODS

A high-throughput analysis of d-deprenyl activity towards 165G-protein coupled receptors and 84 enzyme targets was performed. Additionally, binding studies were used to verify the competition of [(3)H]d-deprenyl with ligands specific for targets identified in the high-throughput screen.

KEY FINDINGS

Our high-throughput investigation identified monoamine oxidase-B, monoamine oxidase-A and angiotensin converting enzyme as potential targets for d-deprenyl. Further competitive [(3)H]d-deprenyl binding studies with specific inhibitors identified monoamine oxidase-B as the major binding site. No evident high-affinity hits were identified among G-protein coupled receptors.

SIGNIFICANCE

Our study was the first to utilize a high-throughput screening approach to identify putative d-deprenyl targets. It verified 249 candidate proteins and confirmed the role of monoamine oxidase - B in d-deprenyl binding. Our results add knowledge about the possible mechanism of d-deprenyl binding, which might aid in explaining the increased uptake of this compound in peripheral inflammation. Monoamine oxidase-B will be further investigated in future studies utilizing human inflamed synovium.

Plasma radiometabolite correction in dynamic PET studies: Insights on the available modeling approaches

Full kinetic modeling of dynamic PET images requires the measurement of radioligand concentrations in the arterial plasma. The unchanged parent radioligand must, however, be separated from its radiometabolites by chromatographic methods. Thus, only few samples can usually be analyzed and the resulting measurements are often noisy. Therefore, the measurements must be fitted with a mathematical model. This work presents a comprehensive analysis of the different models proposed in the literature to describe the plasma parent fraction (PPf) and of the alternative approaches for radiometabolite correction. Finally, we used a dataset of [(11)C]PBR28 brain PET data as a case study to guide the reader through the PPf model selection process.

An exploratory efficacy study of the amyloid imaging agent [(18)F]flutemetamol in Japanese Subjects

AIM

The aim of the study presented was to investigate the brain uptake properties of the amyloid PET agent [(18)F]flutemetamol in Japanese healthy controls and clinically probable Alzheimer's disease (AD) patients, and to make a comparison with the results of a previously performed study on Caucasian subjects. [(18)F]flutemetamol was recently approved by the American Food and Drug Administration and the European Medicines Agency for visualization of amyloid in vivo. Since the first clinical study of [(18)F]flutemetamol-an (18)F derivative of the PET tracer 11C-Pittsburgh Compound B targeting β-amyloid--took place, several clinical studies have been performed, but few focusing on a Japanese population.

METHODS

In the Step A, three elderly healthy volunteers and three AD subjects underwent dynamic PET scanning 0-30 and 60-150 min after injection of 185 MBq [(18)F]flutemetamol. The brain volume of distribution (VT) was quantified using Logan's linear graphical analysis and as standardized uptake value ratios (SUVR) with a cerebellar reference. The optimal acquisition window was determined from brain time activity curves for Step B. In the Step B, 5 AD and 5 elderly healthy volunteers were scanned from 80 to 140 min after intravenous injection of [(18)F]flutemetamol. The data from the two parts were pooled for estimation of overall efficacy.

RESULTS

[(18)F]Flutemetamol injection was shown to be safe-no serious adverse events were reported during this study. A simplified SUVR estimate of the uptake of [(18)F]flutemetamol using a time window of 85-115 min post injection successfully discriminated AD cases from healthy volunteers. AD subjects showed an elevated tracer uptake in prefrontal cortex, the lateral temporal cortex and precuneus amongst other regions. No significant [(18)F]flutemetamol PET differences could be seen between the Japanese AD cases in this study and those from an earlier Caucasian study, or between control subjects in Japanese and Caucasian studies.

CONCLUSIONS

This study supports the use of [(18)F]flutemetamol PET in Japanese population as a marker of the presence of fibrillar β-amyloid. The lack of differences between the Japanese cohort and those from a previous Caucasian cohort supports the extrapolation of results from other Caucasian [(18)F]flutemetamol PET studies to the Japanese population.

Large Variation in Brain Exposure of Reference CNS Drugs: a PET Study in Nonhuman Primates

BACKGROUND

Positron emission tomography microdosing of radiolabeled drugs allows for noninvasive studies of organ exposure in vivo. The aim of the present study was to examine and compare the brain exposure of 12 commercially available CNS drugs and one non-CNS drug.

METHODS

The drugs were radiolabeled with (11)C (t 1/2 = 20.4 minutes) and examined using a high resolution research tomograph. In cynomolgus monkeys, each drug was examined twice. In rhesus monkeys, a first positron emission tomography microdosing measurement was repeated after preadministration with unlabeled drug to examine potential dose-dependent effects on brain exposure. Partition coefficients between brain and plasma (KP) were calculated by dividing the AUC0-90 min for brain with that for plasma or by a compartmental analysis (VT). Unbound KP (KP u,u) was obtained by correction for the free fraction in brain and plasma.

RESULTS

After intravenous injection, the maximum radioactivity concentration (C max, %ID) in brain ranged from 0.01% to 6.2%. For 10 of the 12 CNS drugs, C max, %ID was >2%, indicating a preferential distribution to brain. A lower C max, %ID was observed for morphine, sulpiride, and verapamil. K P ranged from 0.002 (sulpiride) to 68 (sertraline) and 7 of 13 drugs had KP u,u close to unity. For morphine, sulpiride, and verapamil, K P u,u was <0.3, indicating impaired diffusion and/or active efflux. Brain exposure at microdosing agreed with pharmacological dosing conditions for the investigated drugs.

CONCLUSIONS

This study represents the largest positron emission tomography study on brain exposure of commercially available CNS drugs in nonhuman primates and may guide interpretation of positron emission tomography microdosing data for novel drug candidates.

Discovery and development of 11C-Lu AE92686 as a radioligand for PET imaging of phosphodiesterase10A in the brain

Phosphodiesterase 10A (PDE10A) plays a key role in the regulation of brain striatal signaling, and several pharmaceutical companies currently investigate PDE10A inhibitors in clinical trials for various central nervous system diseases. A PDE10A PET ligand may provide evidence that a clinical drug candidate reaches and binds to the target. Here we describe the successful discovery and initial validation of the novel radiolabeled PDE10A ligand 5,8-dimethyl-2-[2-((1-(11)C-methyl)-4-phenyl-1H-imidazol-2-yl)-ethyl]-[1,2,4]triazolo[1,5-a]pyridine ((11)C-Lu AE92686) and its tritiated analog (3)H-Lu AE92686.

METHODS

Initial in vitro experiments suggested Lu AE92686 as a promising radioligand, and the corresponding tritiated and (11)C-labeled compounds were synthesized. (3)H-Lu AE92686 was evaluated as a ligand for in vivo occupancy studies in mice and rats, and (11)C-Lu AE92686 was evaluated as a PET tracer candidate in cynomolgus monkeys and in humans.

RESULTS

(11)C-Lu AE92686 displayed high specificity and selectivity for PDE10A-expressing regions in the brain of cynomolgus monkeys and humans. Similar results were found in rodents using (3)H-Lu AE92686. The binding of (11)C-Lu AE92686 and (3)H-Lu AE92686 to striatum was completely and dose-dependently blocked by the structurally different PDE10A inhibitor 2-[4-(1-methyl-4-pyridin-4-yl-1H-pyrazol-3-yl)-phenoxymethyl]-quinoline (MP-10) in rodents and in monkeys. In all species, specific binding of the radioligand was seen in the striatum but not in the cerebellum, supporting the use of the cerebellum as a reference region. The binding potentials (BPND) of (11)C-Lu AE92686 in the striatum of both cynomolgus monkeys and humans were evaluated by the simplified reference tissue model with the cerebellum as the reference tissue, and BPND was found to be high and reproducible-that is, BPNDs were 6.5 ± 0.3 (n = 3) and 7.5 ± 1.0 (n = 12) in monkeys and humans, respectively.

CONCLUSION

Rodent, monkey, and human tests of labeled Lu AE92686 suggest that (11)C-Lu AE92686 has great potential as a human PET tracer for the PDE10A enzyme.

Kinetic modeling of the monoamine oxidase B radioligand [¹¹C]SL25.1188 in human brain with high-resolution positron emission tomography

This article describes the kinetic modeling of [(11)C]SL25.1188 ([(S)-5-methoxymethyl-3-[6-(4,4,4-trifluorobutoxy)-benzo[d]isoxazol-3-yl]-oxazolidin-2-[(11)C]one]) binding to monoamine oxidase B (MAO-B) in the human brain using high-resolution positron emission tomography (PET). Seven healthy subjects underwent two separate 90- minute PET scans after an intravenous injection of [(11)C]SL25.1188. Complementary arterial blood sampling was acquired. Radioactivity was quickly eliminated from plasma with 80% of parent compound remaining at 90 minutes. Metabolites were more polar than the parent compound. Time-activity curves showed high brain uptake, early peak and washout rate consistent with known regional MAO-B concentration. A two-tissue compartment model (2-TCM) provided better fits to the data than a 1-TCM. Measurement of total distribution volume (VT) showed very good identifiability (based on coefficient of variation (COV)) for all regions of interest (ROIs) (COV(VT)<8%), low between-subject variability (∼20%), and quick temporal convergence (within 5% of final value at 45 minutes). Logan graphical method produces very good estimation of VT. Regional VT highly correlated with previous postmortem report of MAO-B level (r(2)= ≥ 0.9). Specific binding would account from 70% to 90% of VT. Hence, VT measurement of [(11)C]SL25.1(1)88 PET is an excellent estimation of MAO-B concentration.

Modeling of PET data in CNS drug discovery and development

Positron emission tomography (PET) is increasingly used in drug discovery and development for evaluation of CNS drug disposition and for studies of disease biomarkers to monitor drug effects on brain pathology. The quantitative analysis of PET data is based on kinetic modeling of radioactivity concentrations in plasma and brain tissue compartments. A number of quantitative methods of analysis have been developed that allow the determination of parameters describing drug pharmacokinetics and interaction with target binding sites in the brain. The optimal method of quantification depends on the properties of the radiolabeled drug or radioligand and the binding site studied. We here review the most frequently used methods for quantification of PET data in relation to CNS drug discovery and development. The utility of PET kinetic modeling in the development of novel CNS drugs is illustrated by examples from studies of the brain kinetic properties of radiolabeled drug molecules.

Non-input analysis for incomplete trapping irreversible tracer with PET

INTRODUCTION

When using metabolic trapping type tracers, the tracers are not always trapped in the target tissue; i.e., some are completely trapped in the target, but others can be eliminated from the target tissue at a measurable rate. The tracers that can be eliminated are termed 'incomplete trapping irreversible tracers'. These incomplete trapping irreversible tracers may be clinically useful when the tracer β-value, the ratio of the tracer (metabolite) elimination rate to the tracer efflux rate, is under approximately 0.1. In this study, we propose a non-input analysis for incomplete trapping irreversible tracers based on the shape analysis (Shape), a non-input analysis used for irreversible tracers.

METHODS

A Monte Carlo simulation study based on experimental monkey data with two actual PET tracers (a complete trapping irreversible tracer [(11)C]MP4A and an incomplete trapping irreversible tracer [(18)F]FEP-4MA) was performed to examine the effects of the environmental error and the tracer elimination rate on the estimation of the k3-parameter (corresponds to metabolic rate) using Shape (original) and modified Shape (M-Shape) analysis. The simulation results were also compared with the experimental results obtained with the two PET tracers.

RESULTS

When the tracer β-value was over 0.03, the M-Shape method was superior to the Shape method for the estimation of the k3-parameter. The simulation results were also in reasonable agreement with the experimental ones.

CONCLUSIONS

M-Shape can be used as the non-input analysis of incomplete trapping irreversible tracers for PET study.

Extraction of an input function from dynamic micro-PET images using wavelet packet based sub-band decomposition independent component analysis

Positron emission tomography (PET) can be used to quantify physiological parameters. However to perform quantification requires that an input function is measured, namely a plasma time activity curve (TAC). Image-derived input functions (IDIFs) are attractive because they are noninvasive and nearly no blood loss is involved. However, the spatial resolution and the signal to noise ratio (SNR) of PET images are low, which degrades the accuracy of IDIFs. The objective of this study was to extract accurate input functions from microPET images with zero or one plasma sample using wavelet packet based sub-band decomposition independent component analysis (WP SDICA). Two approaches were used in this study. The first was the use of simulated dynamic rat images with different spatial resolutions and SNRs, and the second was the use of dynamic images of eight Sprague-Dawley rats. We also used a population-based input function and a fuzzy c-means clustering approach and compared their results with those obtained by our method using normalized root mean square errors, area under curve errors, and correlation coefficients. Our results showed that the accuracy of the one-sample WP SDICA approach was better than the other approaches using both simulated and realistic comparisons. The errors in the metabolic rate, as estimated by one-sample WP SDICA, were also the smallest using our approach.

A method to predict the ratio of the tracer conversion rate to the tracer back-diffusion rate of an irreversible-type radiotracer in humans by preclinical evaluations

OBJECTIVE

The aim of this study was to develop a method to predict a tracer's α-value in the human brain on the basis of animal data. The α-value is the ratio of the conversion rate and the back-diffusion rate (k3/k2) and is one of the critical kinetic features of the detection sensitivity of target molecule activity, such as enzyme activity, in the measurement of PET and single-photon emission computed tomography using an irreversible-type radiotracer.

METHOD

The α-value in the rat brain was estimated by a simultaneous assay of the tracer uptake and the target biochemical activity using N-[C]-methylpiperidin-4-yl acetate ([C]MP4A) and N-[C]-methylpiperidin-4-yl propionate ([C]MP4P) as test tracers, both of which are metabolic trapping tracers for measurement of brain acetylcholinesterase. The α-value in humans was then extrapolated from the α-value in rats by considering the differences between the species. The predicted human α-values were compared with those obtained from the kinetic analyses of human PET studies using [C]MP4A and [C]MP4P.

RESULT

The α-values in the human brain cortex were predicted to be 0.51±0.1 for MP4A and 0.25±0.05 for MP4P. These results were close to values reported in other PET studies: 0.48±0.1 to 0.73±0.2 for MP4A and 0.15±0.04 to 0.18±0.04 for MP4P.

CONCLUSION

The α-value predicted by this method would be used for practical selection or development of irreversible-type radiotracers for human use.

Activated MAO-B in the brain of Alzheimer patients, demonstrated by [11C]-L-deprenyl using whole hemisphere autoradiography

In the human brain the monoaminooxidase-B enzyme or MAO-B is highly abundant in astrocytes. As astrocyte activity and, consequently, the activity of the MAO-B enzyme, is up-regulated in neuroinflammatory processes, radiolabelled analogues of deprenyl may serve as an imaging biomarker in neuroinflammation and neurodegeneration, including Alzheimer's disease. In the present study [(11)C]-L-deprenyl, the PET radioligand version of L-deprenyl or selegiline®, a selective irreversible MAO-B inhibitor was used in whole hemisphere autoradiographic experiments in human brain sections in order to test the radioligand's binding to the MAO-B enzyme in human brain tissue, with an eye on exploring the radioligand's applicability as a molecular imaging biomarker in human PET studies, with special regard to diagnostic detection of reactive astrogliosis. Whole hemisphere brain sections obtained from Alzheimer patients and from age matched control subjects were examined. In control brains the binding of [(11)C]-L-deprenyl was the highest in the hippocampus, in the basal ganglia, the thalamus, the substantia nigra, the corpus geniculatum laterale, the nucleus accumbens and the periventricular grey matter. In Alzheimer brains significantly higher binding was observed in the temporal lobes and the white matter. Furthermore, in the Alzheimer brains in the hippocampus, temporal lobe and white matter the binding negatively correlated with Braak stages. The highest binding was observed in Braak I-II, whereas it decreased with increasing Braak grades. The increased regional binding in Alzheimer brains coincided with the presence of an increased number of activated astrocytes, as demonstrated by correlative immunohistochemical studies with GFAP in adjacent brain slices. Deprenyl itself as well as the MAO-B antagonist rasagiline did effectively block the binding of the radioligand, whereas the MAO-A antagonist pirlindole did not affect it. Compounds with high affinity for the PBR system did not block the radioligand binding either, providing evidence for the specificity of [(11)C]-L-deprenyl for the MAO-B enzyme. In conclusion, the present observations indicate that [(11)C]-L-deprenyl may be a promising and selective imaging biomarker of increased MAO-B activity in the human brain and can therefore serve as a prospective PET tracer targeting neuroinflammation and neurodegeneration.

In vivo quantification of monoamine oxidase A in baboon brain: a PET study using [(11)C]befloxatone and the multi-injection approach

[(11)C]befloxatone is a high-affinity, reversible, and selective radioligand for the in vivo visualization of the monoamine oxidase A (MAO-A) binding sites using positron emission tomography (PET). The multi-injection approach was used to study in baboons the interactions between the MAO-A binding sites and [(11)C]befloxatone. The model included four compartments and seven parameters. The arterial plasma concentration, corrected for metabolites, was used as input function. The experimental protocol-three injections of labeled and/or unlabeled befloxatone-allowed the evaluation of all the model parameters from a single PET experiment. In particular, the brain regional concentrations of the MAO-A binding sites (B'(max)) and the apparent in vivo befloxatone affinity (K(d)) were estimated in vivo for the first time. A high binding site density was found in almost all the brain structures (170+/-39 and 194+/-26 pmol/mL in the frontal cortex and striata, respectively, n=5). The cerebellum presented the lowest binding site density (66+/-13 pmol/mL). Apparent affinity was found to be similar in all structures (K(d)V(R)=6.4+/-1.5 nmol/L). This study is the first PET-based estimation of the B(max) of an enzyme.

Quantitative analysis of donepezil binding to acetylcholinesterase using positron emission tomography and [5-(11)C-methoxy]donepezil

The aim of this study was to establish kinetic analysis of [5-(11)C-methoxy]donepezil ([(11)C]donepezil), which was developed for the in-vivo visualization of donepezil binding to acetylcholinesterase (AChE) using positron emission tomography (PET). Donepezil is an AChE inhibitor that is widely prescribed to ameliorate the cognitive impairment of patients with dementia. Six healthy subjects took part in a dynamic study involving a 60-min PET scan after intravenous injection of [(11)C]donepezil. The total distribution volume (tDV) of [(11)C]donepezil was quantified by compartmental kinetic analysis and Logan graphical analysis. A one-tissue compartment model (1TCM) and a two-tissue compartment model (2TCM) were applied in the kinetic analysis. Goodness of fit was assessed with chi(2) criterion and Akaike's Information Criterion (AIC). Compared with a 1TCM, goodness of fit was significantly improved by a 2TCM. The tDVs provided by Logan graphical analysis were slightly lower than those provided by a 2TCM. The rank order of the mean tDVs in 10 regions was in line with the AChE activity reported in a previous post-mortem study. Logan graphical analysis generated voxel-wise images of tDV, revealing the overall distribution pattern of AChE in individual brains. Significant correlation was observed between tDVs calculated with and without metabolite correction for plasma time-activity curves, indicating that metabolite correction could be omitted. In conclusion, this method enables quantitative analysis of AChE and direct investigation of the pharmacokinetics of donepezil in the human brain.

A novel blood-cell-two-compartment model for transferring a whole blood time activity curve to plasma in rodents

The term input function usually refers to the tracer plasma time activity curve (pTAC), which is necessary for quantitative positron emission tomography (PET) studies. The purpose of this study was to acquire the pTAC by independent component analysis (ICA) estimation from the whole blood time activity curve (wTAC) using a novel method, namely the FDG blood-cell-two-compartment model (BCM). This approach was compared to a number of published models, including linear haematocrit (HCT) correction, non-linear HCT correction and two-exponential correction. The results of this study show that the normalized root mean square error (NRMSE) and the error of the area under curve (EAUC) for the BCM estimate of the pTAC were the smallest. Compartmental and graphic analyses were used to estimate the metabolic rate of the FDG (MR(FDG)). The percentage error for the MR(FDG) (PE(MRFDG)) was estimated from the BCM corrected pTAC and this was also the smallest. It is concluded that the BCM is a better choice when transferring wTAC into pTAC for quantification.

Monoamine oxidase B activity is increased in human gliomas

Glial tumours are the most common type of brain neoplasm in humans. Tumour classification and grading represent key factors for patient management. However, current grading schemes are still limited by subjective histological criteria. In this context, gliosis has been linked to increases in monoamine oxidase B (MAO-B) activity. Thus, in the present study, MAO-B activity in membranes of glial tumours (n=20), meningiomas (n=12) and non-pathological human brains (n=15) was quantified by [14C]PEA oxidation. MAO-B activity was significantly greater in glioblastoma multiformes than in postmortem control brains (p<0.01) or meningiomas (p<0.001). There were no significant differences in MAO-B activity between glioblastoma multiformes (n=11) and low-grade astrocytomas (n=3) or anaplastic astrocytomas (n=6). In conclusion, the present results demonstrate a significant and selective increase in MAO-B activity in human gliomas when compared with meningiomas or non-tumoural tissue. These results suggest that the quantification of MAO-B activity may be a useful diagnostic tool for differentiating glial tumours from other types of brain tumours or surrounding normal brain tissue.

Benzodiazepine-GABA(A) receptor binding is very low in dysembryoplastic neuroepithelial tumor: a PET study

PURPOSE

To determine the nature of abnormalities of gamma-aminobutyric acid (GABA)A-central benzodiazepine (BZD) receptor binding in patients with dysembryoplastic neuroepithelial tumor (DNET) in comparison with normal controls.

METHODS

Five patients with DNET and 24 normal controls underwent (11C)flumazenil positron emission tomography (PET) to measure (11C)flumazenil volume of distribution (FMZVD) at the voxel level. Patients were compared with normal controls by using statistical parametric mapping (SPM) and also a partial-volume effect (PVE) corrected volume-of-interest (VOI) analysis covering the entire brain. First, using SPM, the highest Z-score for the entire image representing FMZVD decreases in comparison with the normals was found. Second, regions of abnormal FMZVD were located using SPM, p < 0.001 uncorrected, corrected p < 0.05. Finally, PVE-corrected measures of FMZVD were calculated for each patient VOI and compared wih those of normals, using significance levels of >2.5 standard deviations (SD) for the DNET and >3 SD for all other regions.

RESULTS

In all cases, the highest Z-score across the whole image representing decreased FMZVD was within the DNET. In three cases SPM revealed a single region of significantly reduced FMZVD, within the DNET in all three. VOI analysis showed PVE-corrected FMZVD was significantly low in the DNET in four cases. VOI analysis also showed seven other regions of abnormal FMZVD; three were adjacent to a DNET, and two were in mesial temporal areas not affected by DNET.

CONCLUSIONS

FMZVD is low in DNET, probably contributing to epileptogenicity.

Temporal lobe epilepsy visualized with PET with 11C-L-deuterium-deprenyl--analysis of kinetic data

OBJECTIVES

The purpose of the study was to develop a simplified method for the acquisition and analysis of data from positron emission tomography (PET) using the ligand 11C-L-deuterium-deprenyl. This is motivated by an increased interest in methods to characterize gliosis in neurodegenerative diseases and epilepsy, which can be defined due to an increased expression of the enzyme MAO-B.

METHODS

Seven patients with temporal lobe epilepsy were investigated with PET. The tracer kinetics in different brain structures was recorded and analyzed using different models with and without a plasma input function. The derived values were correlated to literature values of 3H-deprenyl binding in frozen sections from normal human brains.

RESULTS

A good correlation was seen between in vivo binding and in vitro data, with the correlation being equally good irrespective of whether metabolite corrected plasma or modified cerebellar uptake values were used as input function. The epileptic lobe was, compared to non-epileptic, characterized by a lower initial distribution and an enhanced late accumulation of the tracer. With the applied method, it was possible to correctly identify the epileptic side in all 6 unilateral patients and I probable bilateral case.

CONCLUSIONS

PET with 11C-L-deuterium-deprenyl gives a good correlation between calculated in vivo binding and MAO-B activity. The analysis can be simplified and blood sampling avoided if modified cerebellar time-activity data is used as a reference. Separate images of distribution volume and MAO-B binding can be generated.

Central benzodiazepine/gamma-aminobutyric acid A receptors in idiopathic generalized epilepsy: an [11C]flumazenil positron emission tomography study

PURPOSE

Previous [11C]flumazenil (FMZ) positron emission tomography (PET) investigations in patients with idiopathic generalized epilepsy (IGE) have demonstrated nonsignificant global cortical decreases in central benzodiazepine gamma-aminobutyric acid A (GABA[A]) receptor (cBZR) binding or focal decreases in the thalamus and increases in the cerebellar nuclei with no changes in cerebral cortex. We previously reported lower [11C]FMZ binding in cerebral cortex of IGE patients treated with valproate (VPA) than in cerebral cortex of controls. We now report high-resolution three-dimensional [11C]FMZ PET studies in a larger number of subjects using an improved method to detect differences in cBZR between IGE patients and controls and a more powerful longitudinal design to determine the functional effect of VPA.

METHODS

We compared parametric images of [11C]FMZ volume of distribution (FMZVD) in 10 IGE patients before and after addition of VPA and in 20 normal subjects.

RESULTS

Mean FMZVD was significantly higher in the cerebral cortex (11%, p = 0.009), thalamus (14%, p = 0.018), and cerebellum (15%, p = 0.027) of the 10 IGE patients as compared with that of 20 normal controls. Using statistical parametric mapping, no significant areas of focal abnormality of FMZVD were detected. Addition of VPA was not associated with a significant change in mean FMZVD in any brain area.

CONCLUSIONS

Our finding of increased FMZVD in IGE could reflect microdysgenesis or a state of cortical hyperexcitability. Our data suggest that short-term VPA therapy does not affect the number of available cBZR in patients with IGE.

Synthesis of some 11C-labelled MAO-A inhibitors and their in vivo uptake kinetics in rhesus monkey brain

Five potential MAO-A inhibitors--harmine, N-methyl-harmine, harmaline, brofaromine, and clorgyline--were labelled with 11C and their brain kinetics evaluated in vivo in rhesus monkey using PET. The compounds were synthesized by alkylation with 11C methyl iodide and obtained in 48-89% radiochemical yield within 40 to 45 min synthesis time and with specific radioactivities in the region of 0.49-2.4 Ci mumol-1 (18-87 GBq mumol-1) at the end of synthesis. The kinetic pattern after administration of MAO-A inhibitors was comparable to that seen in the tracer study when using 11C-brofaromine, 11C-harmaline, or 11C-clorgyline, although the magnitude of uptake markedly increased in the case of brofaromine and harmaline. Both 11C-methylharmine and 11C-harmine showed a significant washout in the inhibition studies. The kinetics of brain uptake with and without MAO-A inhibition is compatible with a significant fraction of the tracer bound to MAO-A for 11C-methylharmine and 11C-harmine, whereas 11C-brofaromine, 11C-harmaline, or 11C-clorgyline did not seem to show specific enzyme binding.

11C-harmine as a tracer for monoamine oxidase A (MAO-A): in vitro and in vivo studies

Frozen-section autoradiography in rat brain sections as well as in vivo positron emission tomography (PET) studies in monkey brain were used for the determination of binding characteristics of O-[methyl-11C]harmine in an attempt to validate this ligand for the assessment of monoamine oxidase A (MAO-A). In frozen sections, the binding of [11C]harmine showed an apparent KD of the binding of 2 nM. The specific binding was inhibited by nanomolar concentrations of clorgyline, esuprone, brofaromine, and Ro 41-1049. The in vivo kinetic pattern in the monkey brain indicated a significant trapping, which was inhibited by pretreatment with clorgyline, moclobemide, or harmine. Different approaches for a quantitative determination of MAO-A enzyme binding were attempted and demonstrated an IC50 dose of harmine in the range of 0.05-0.1 mg/kg. The studies give strong indications for the validity of [11C]harmine as an in vivo tracer for the assessment of MAO-A enzyme binding in the brain.

Analysis of dynamic radioligand displacement or "activation" studies

We present a simple way of assessing dynamic or time-dependent changes in displacement during single-subject radioligand positron emission tomography (PET) activation studies. The approach is designed to facilitate dynamic activation studies using selective radioligands. These studies are, in principle, capable of characterising functional neurochemistry by analogy with the study of functional neuroanatomy using rCBF activation studies. The proposed approach combines time-dependent compartmental models of tracer kinetics and the general linear model used in statistical parametric mapping. This provides for a comprehensive, voxel-based and data-led assessment of regionally specific effects. The statistical model proposed in this paper is predicated on a single-compartment model extended to allow for time-dependent changes in kinetics. We have addressed the sensitivity and specificity of the analysis, as it would be used operationally, by applying the analysis to 11C-Flumazenil dynamic displacement studies. The activation used in this demonstration study was a pharmacological (i.v. midazolam) challenge, 30 min after administration of the tracer. We were able to demonstrate, and make statistical inferences about, regional increases in k2 (or decreases in the volume of distribution) in prefrontal and other cortical areas.

Cardiac and skeletal muscle insulin resistance in patients with coronary heart disease. A study with positron emission tomography

Patients with coronary artery disease or heart failure have been shown to be insulin resistant. Whether in these patients heart muscle participates in the insulin resistance, and whether reduced blood flow is a mechanism for such resistance is not known. We measured heart and skeletal muscle blood flow and glucose uptake during euglycemic hyperinsulinemia (insulin clamp) in 15 male patients with angiographically proven coronary artery disease and chronic regional wall motion abnormalities. Six age- and weight-matched healthy subjects served as controls. Regional glucose uptake was measured by positron emission tomography using [18F]2-fluoro-2-deoxy-D-glucose (FDG), blood flow was measured by the H2(15)O method. Myocardial glucose utilization was measured in regions with normal perfusion and wall motion as assessed by radionuclide ventriculography. Whole-body glucose uptake was 37+/-4 micromol x min(-1) x kg(-1) in controls and 14+/-2 mciromol x min(-1) x kg(-1) in patients (P = 0.001). Myocardial blood flow (1.09+/-0.06 vs. 0.97+/-0.04 ml x min(-1) x g(-1), controls vs. patients) and skeletal muscle (arm) blood flow (0.046+/-0.012 vs. 0.043+/-0.006 ml x min(-1) x g(-1)) were similar in the two groups (P = NS for both). In contrast, in patients both myocardial (0.38+/-0.03 vs. 0.70+/-0.03 micromol x min(-1) x g(-1), P = 0.0005) and muscle glucose uptake (0.026+/-0.004 vs. 0.056+/-0.006 micromol x min(-1) x g(-1), P = 0.005) were markedly reduced in comparison with controls. In the whole dataset, a direct relationship existed between insulin-stimulated glucose uptake in heart and skeletal muscle. Patients with a history of myocardial infarction and a low ejection fraction are insulin resistant. This insulin resistance affects both the myocardium and skeletal muscle and is independent of blood flow.

Benzodiazepine receptors in focal epilepsy with cortical dysgenesis: an 11C-flumazenil PET study

Previous imaging studies using 11C-flumazenil in patients with mesial temporal lobe epilepsy and neocortical partial seizure disorders have found focal decreases in gamma-aminobutyric acid type A/benzodiazepine receptor binding. These studies used subjective visual assessment and a region of interest approach to quantitation. We performed three-dimensional, 11C-flumazenil positron emission tomography in 12 patients with cortical dysgenesis identified by high-resolution volumetric magnetic resonance imaging and in 26 normal subjects. Spectral analysis was used to produce a parametric image of 11C-flumazenil volume of distribution for each subject. Using volumetric normalization and statistical parametric mapping, we compared the entire brain volume of each patient with the brains of the normal group to produce maps of regions of abnormal 11C-flumazenil binding which were then rendered into the volumetric magnetic resonance images. This allowed a correlation of structure and function to be made. Of the 12 patients, 10 showed at least one region of abnormal 11C-flumazenil binding; the abnormal regions were frequently more extensive than were the lesions seen with magnetic resonance imaging. 11C-Flumazenil binding abnormalities were frequently seen in regions of cortex that had a normal magnetic resonance appearance. Lesions were characterized by increases in gamma-aminobutyric acid type A/benzodiazepine receptor availability, and by the decreases found in previous studies. These findings have implications for the neurobiology of seizure disorders associated with cortical dysgenesis and for the management of such patients if surgery is contemplated.

Pathophysiology of chronic left ventricular dysfunction. New insights from the measurement of absolute myocardial blood flow and glucose utilization

BACKGROUND

Chronically dysfunctional myocardium may improve after coronary revascularization. This condition was thought to be due to a chronically reduced myocardial blood flow (MBF). Recently, however, it has been shown that in patients without previous infarction but with chronic left ventricular dysfunction, baseline MBF was normal.

METHODS AND RESULTS

To study the pathophysiology of chronic left ventricular dysfunction in patients with previous infarction, regional MBF (milliliter per minute per gram of water-perfusable tissue) and glucose utilization (MRG; micromoles per minute per gram) during hyperinsulinemic euglycemic clamp were measured with positron emission tomography in 30 patients before bypass. At baseline, 133 myocardial segments were normal, and 107 were dysfunctional. After revascularization, 59 of 107 segments improved, while 48 of 107 were unchanged. MBF was 0.92 +/- 0.25 mL.min-1.g-1 in normal segments, 0.87 +/- 0.31 mL.min-1.g-1 in improved segments (P = NS versus normal), and 0.82 +/- 0.40 mL.min-1.g-1 in unchanged segments (P < .05 versus normal). In 90% of the dysfunctional segments, MBF was > 0.42 mL.min-1.g-1, a cutoff value corresponding to the mean MBF minus 2 SD in normal segments. The MRG was 0.71 +/- 0.14 mumol.min-1.g-1 in 9 age-matched normal subjects, 0.45 +/- 0.19 mumol.min-1.g-1 (P < .01) in normal segments, 0.44 +/- 0.14 mumol.min-1.g-1 in improved segments (P = NS versus normal), and 0.34 +/- 0.17 mumol.min-1.g-1 in unchanged segments (P < .01 versus normal and improved).

CONCLUSIONS

The results suggest that resting MBF measured with 15O-labeled water in chronically dysfunctional segments is not reduced and that the myocardium of these patients is less sensitive to insulin than that of normal subjects.

Synthesis and in vivo studies of a specific monoamine oxidase B inhibitor: 5-[4-(benzyloxy)phenyl]-3-(2-cyanoethyl)- 1,3,4-oxadiazol-[11C]-2(3H)-one

We report the radiochemical synthesis of a specific MAO B inhibitor, namely 5-[4-(benzyloxy)phenyl]-3-(2-cyanoethyl)-1,3,4-oxadiazol-[11C]-2(3 H)-one (2b) (in vitro IC50=4nM and selectivity over 71000 for MAO B), by cyclization of its hydrazide precursor 1 with [11C]phosgene. The reaction occurred within 2 min. The product obtained after HPLC purification, 2b, had a high specific activity (11.1-22.2 GBq/micromol), allowing its use in experiments as a radiotracer in vivo. Biodistribution of 2b in the CNS and in the peripheral organs of the rat, and positron emission tomography (PET) studies in the living baboon brain, pretreated or not with L-deprenyl (1mg/kg, 1h), an irreversible MAO B-specific inhibitor, were undertaken. The results showed a good uptake of 2b in all organs of the rat, with a rapid clearance from the blood (10 min). Metabolite analyses in plasma and in the diencephalon of the rat showed that 2b was the only radioactive compound in brain structure whereas in plasma three other radioactive products appeared. PET experiments show that in the L-deprenyl-pretreated baboon brain, specific binding of 2b represents around 70% of total radioactivity, whereas in the blood and plasma the radioactivity cleared rapidly (15 min).

Comparison of methods for analysis of clinical [11C]raclopride studies

Five different methods for the estimation of the binding potential, a measure of Bmax/Kd, of [11C]raclopride in human striatum were compared using data from a dose ranging study of the neuroleptic CP-88,059-01. Binding potential was estimated indirectly, from distribution volumes in striatum and cerebellum, using both single- and two-tissue compartment models with a metabolite-corrected plasma curve as input function. The two-tissue compartment model was also used for a direct estimate of the binding potential. In addition, a direct estimate was obtained from the reference tissue compartment model using the cerebellum as indirect input function. Finally, an estimate of binding potential was calculated from the ratio of striatum over cerebellum counts at late times after injection. The estimates of striatum binding potential from all methods, except the direct determination using a two-tissue compartment model with metabolite-corrected plasma input function, correlated with each other. Use of an average metabolite correction resulted in only a small reduction in accuracy in this series of normal subjects. The reference tissue model provided estimates of the binding potential with the same sensitivity for detecting changes as those methods that required a metabolite-corrected plasma input function. This indicates that for routine analysis of clinical [11C]raclopride studies, no arterial cannulation is required. The range of normal values was significantly less variable with the reference tissue method than when simple striatum-to-cerebellum ratios were used.

Effect of tissue heterogeneity on quantification in positron emission tomography

As a result of the limited spatial resolution of positron emission tomographic scanners, the measurements of physiological parameters are compromised by tissue heterogeneity. The effect of tissue heterogeneity on a number of parameters was studied by simulation and an analytical method. Five common tracer models were assessed. The input and tissue response functions were assumed to be free from noise and systematic errors. The kinetic model was assumed to be perfect. Two components with different kinetics were mixed in different proportions and contrast with respect to the model parameters. Different experimental protocols were investigated. Of three methods investigated for the measurement of cerebral blood flow (CBF) (steady state, dynamic, integral), the second one was least sensitive to errors caused by tissue heterogeneity and the main effect was an underestimation of the distribution volume. With the steady state method, errors in oxygen extraction fraction caused by tissue heterogeneity were always found to be less than the corresponding errors in CBF. For myocardial blood flow the steady state method was found to perform better than the bolus method. The net accumulation of substrate (i.e. rCMRglc in the case of glucose analogs) was found to be comparatively insensitive to tissue heterogeneity. Individual rate constants such as k2 and k3 for efflux and metabolism of the substrate in the pool of unmetabolized substrate in the tissue, respectively, were found to be more sensitive. In studies of radioligand binding, using only tracer doses, the effect of tissue heterogeneity on the parameter kon.Bmax could be considerable. In studies of radioligand binding using a protocol with two experiments, one with high and one with low specific activity, Bmax was found to be insensitive while Kd was very sensitive to tissue heterogeneity.

Benzodiazepine-GABAA receptor binding during absence seizures

The role of benzodiazepine (BZD)-gamma-aminobutyric acidA (GABAA) receptors in the pathogenesis of absence seizures is uncertain. In this study, we examined the effect of absence seizures on the binding of flumazenil to the BZD binding site of the GABAA receptor. Five patients with idiopathic generalized epilepsy (IGE) were studied at rest and during absence seizures with [11C]flumazenil and positron emission tomography (PET). Normalized regional cerebral time-activity curves from the resting and ictal scans were compared with each other and with computed simulations showing the effects of changes in cerebral blood flow (CBF) and [11C]flumazenil binding. No evidence was found for a change in [11C]flumazenil binding with absence seizures. This result, together with those of a recent study showing no abnormality of [11C]flumazenil binding interictally in patients with childhood and juvenile absence epilepsy (JAE) does not support a primary role for the BZD binding site of the GABAA receptor in the pathogenesis of absence seizures.

Benzodiazepine-GABAA receptors in idiopathic generalized epilepsy measured with [11C]flumazenil and positron emission tomography

The neurochemical basis of absence seizures and the mechanism of their suppression by valproate (VPA) are uncertain. We used positron emission tomography (PET) to determine whether an abnormality of [11C]flumazenil binding to benzodiazepine (BZD)-GABAA receptors exists in patients with childhood and juvenile absence epilepsy and to examine the effects of VPA on [11C]flumazenil binding. The regional cerebral volume of distribution (Vd) of [11C]flumazenil in patients not treated with VPA was not different from that in normal controls; Vd was lower in patients treated with VPA, and the number of receptors available for binding was significantly reduced in such patients as compared with normal controls. There was no evidence of a primary abnormality of the BZD-GABAA receptor in childhood and juvenile absence epilepsy (CAE/JAE), but the data suggest that treatment with VPA is associated with a reduction in [11C]flumazenil binding that may be relevant to its mode of action in CAE/JAE.

Imaging the head: functional imaging

Images

Benzodiazepine receptor quantification in vivo in humans using [11C]flumazenil and PET: application of the steady-state principle

Carbon-11-labeled flumazenil combined with positron emission tomography (PET) was used to measure the concentration (Bmax) of the benzodiazepine (Bz) receptor in the brain and its equilibrium dissociation constant (KD) for flumazenil in five normal subjects. The steady-state approach was used injecting the tracer as a bolus of high specific activity. In each subject two studies were carried out. The first study was performed at essentially zero receptor occupancy, the tracer alone study. The second study was performed at a steady-state receptor occupancy of about 50%, achieved by a prolonged constant infusion of nonlabeled ("cold") flumazenil starting 2h before the bolus tracer injection and continuing until the end of scanning period. In this second study the free concentration of unmetabolized flumazenil in plasma water was measured in multiple blood samples. The observed tissue and plasma tracer curves, calibrated in the same units of radioactivity per millimeter, were analyzed in two ways: (a) by the noncompartmental (stochastic) approach making no assumptions regarding number of compartments in the tissue, and (b) by the single-compartment approach assuming rapid exchange (mixing) of tracer between all tissue compartments. The noncompartmental and the compartmental analyses gave essentially the same values for the distribution volume of the tracer, the parameter used for quantitation of the Bz receptor. As the compartmental approach could be applied to a shorter observation period (60 min instead of 120 min) it was preferred. The five subjects had a mean KD value of 12 nM/L of water and Bmax values of the grey matter ranging from 39 +/- 11 in thalamus to 120 +/- 14 nM/L of brain in occipital cortex. Most previous studies have been based on the pseudoequilibrium approach using the brain stem as a receptor-free reference region. This yields practically the same KD but lower Bmax values than the steady-state approach presented here.

Quantification of in vivo binding of [3H]RX 821002 in rat brain: evaluation as a radioligand for central alpha 2-adrenoceptors

On the basis of its established in vitro characteristics, [3H]RX 821002 was evaluated in rats as an in vivo radioligand for central alpha 2-adrenoceptors. Estimates for in vivo binding potential, obtained by compartmental analyses of time-radioactivity data, ranged between 1.9 for hypothalamus and 0.2 for cerebellum, with a regional distribution in brain which was similar to that observed in vitro. Selectivity and specificity of the signal were checked by predosing with either the alpha 2-antagonists, idazoxan or yohimbine, the alpha 2-agonist, clonidine, or the alpha 1-antagonist, prazosin. Pretreatment of the rats with the selective neurotoxin, DSP-4, had no significant effect on [3H]RX 821002 binding, suggesting that the majority of labelled sites were situated post-junctionally. The studies indicate that [3H]RX 821002 can be used experimentally as an in vivo marker for central alpha 2-adrenoceptors. The size and rate of expression of the specific signal encourage the development and assessment of [11C]RX 821002 for clinical PET studies.

Evaluation of S-[11C]citalopram as a radioligand for in vivo labelling of 5-hydroxytryptamine uptake sites

The biologically active S-enantiomer of [N-methyl-11C]citalopram was evaluated as a radioligand for in vivo labelling of the 5-hydroxytryptamine uptake site in brain, using ex vivo tissue counting in rats and positron emission tomography in man. In rats, the maximal signal for total versus non-specific binding was approx. 2 at 60-120 min after radioligand injection. Subsequent studies in man failed to identify a specific signal over a 90 min scanning period, due to prolonged retention of non-specific label.

Quantitation of carbon-11-labeled raclopride in rat striatum using positron emission tomography

Using conventional autoradiographic and tissue counting techniques, the experimental quantitation of in vivo kinetics of prospective or established radioligands for PET is animal and labour intensive. The present study tested the feasibility of using PET itself to quantitate the specific binding of [11C]raclopride to rat striatum and to study the effects of experimental manipulation of endogenous dopamine on binding parameters. Carbon-11-labeled raclopride was given i.v. to anaesthetised rats, positioned in a PET camera and dynamic emission scans acquired over 60 min. Time-activity curves were generated for selected regions of interest, representing striatum and cerebellum and the striatal data fitted to a compartmental model, using cerebellum as the input function, thus circumventing the need for individual metabolite-corrected plasma curves. In control rats, the binding potential (BP), defined as the ratio of the rate constants for transfer from "free to bound" and "bound to free" compartments, was of the order of 0.6. This was reduced threefold by predosing with nonradioactive raclopride. Increasing extracellular dopamine levels by predosing with d-amphetamine resulted in a significant decrease in BP whereas reducing extracellular dopamine by predosing with gamma-butyrolactone caused a significant increase. Thus, despite the limitation in spatial resolution of PET, specific binding of raclopride could be assessed from regional time-activity curves from individual rats. The system was sufficiently sensitive that changes in BP could be detected following modulation of endogenous dopamine levels, a finding of potential relevance to the interpretation of clinical PET data.

A convenient method for regional monoamine oxidase-A determination by [14C]clorgyline autoradiography

The availability of clorgyline for regional monoamine oxidase-A (MAO-A) determination was examined using [14C]clorgyline in rat. [14C]Clorgyline was synthesized by the methylation reaction of N-desmethyl-clorgyline and [14C]methyliodide in dimethylformamide with high radiochemical yield. The MAO-A distribution map by autoradiography correlated with that by histochemical technique and its quantity was consistent with the calculated MAO-A amount based on previous reports. The combination of labeled clorgyline and autoradiographic technique will promise the quantitative measurement of regional MAO-A distribution.