Absorbed fractions for dose calculations of neuroreceptor PET studies

A simple Monte Carlo program was constructed for the purpose of calculating absorbed fractions of relevance to neurologic positron-emission-tomography studies. The caudate, putamen, and cerebellum regions of the brain were mathematically modeled to serve as source regions for the absorbed-fraction calculations for positron-annihilation photons. The target organs were the caudate, putamen, cerebellum, brain, spine portion in the head, skull, and head. Absorbed fractions were also calculated for a uniform ring source which encircled the head phantom.     

Absorbed fractions for dose calculations of neuroreceptor PET studies

A simple Monte Carlo program was constructed for the purpose of calculating absorbed fractions of relevance to neurologic positron-emission-tomography studies. The caudate, putamen, and cerebellum regions of the brain were mathematically modeled to serve as source regions for the absorbed-fraction calculations for positron-annihilation photons. The target organs were the caudate, putamen, cerebellum, brain, spine portion in the head, skull, and head. Absorbed fractions were also calculated for a uniform ring source which encircled the head phantom.     

Simplified detection system for neuroreceptor studies in the human brain

A simple, inexpensive dual-detector system has been developed for measurement of positronemitting receptor-binding drugs in the human brain. This high efficiency coincidence counting system requires that only a few hundred microcuries of labeled drug be administered to the subject, thereby allowing for multiple studies without an excessive radiation dose. Measurement of the binding of [11C]carfentanil, a high affinity synthetic opiate, to opiate receptors in the presence and in the absence of a competitive opiate antagonist indicates the potential utility of this system for estimating different degrees of receptor occupation in the human brain.     

Motion artifact reduction on parametric PET images of neuroreceptor binding.

PET studies of cerebral neuroreceptors are often recorded over periods ranging from 1 to 2 h, and head movements during the studies not only lead to blurred images but also may seriously disturb the kinetic analysis. We report the effect of motion on parametric images of the distribution volume ratio (DVR), as well as possible improvements if the dynamic PET data are corrected for head movements. METHODS: The study was performed with the 5-hydroxytryptamine 2A receptor ligand (18)F-altanserin. During PET scanning, which was performed in list mode for 1 h, the position of the head was monitored by an infrared motion-tracking system. The list mode data were sorted into time frames of between 10 s and 2 min. Motion was corrected using the multiple-acquisition-frame (MAF) approach, which calculates individual attenuation files for each emission frame and its corresponding head position to avoid misalignment of transmission and emission data. After reconstruction of attenuation-corrected emission frames, each image frame was realigned to match the head position of the first frame of the emission scan. The resulting motion-corrected dynamic images were evaluated using the noninvasive Logan plot to obtain parametric images of DVR. RESULTS: DVR images of motion-affected (18)F-altanserin scans showed artifacts whose extent depended on the amount of movement. The artifacts were mainly at the border between gray matter and white matter and at the outer border of gray matter. They were seen as discontinuities and small spots whose values exceeded the expected DVR values or were even negative and that disappeared when motion correction was applied. These effects in human data were also seen on simulated (18)F-altanserin images that contained no statistical noise. CONCLUSION: Whereas the native PET images looked just blurred if the patient moved during the PET scan, parametric images of the Logan DVR, which are calculated by pixelwise linear regression, contained severe discontinuities primarily at the cortical edge. MAF-based motion correction was able to avoid these errors.     

Effects of statistical noise on graphic analysis of PET neuroreceptor studies.

Because of its computational simplicity, the graphic method introduced by Logan et al. is frequently used to analyze time-activity curves of reversible radiotracers measured in brain regions with PET. The graphic method uses a nonlinear transformation of data to variables that have an asymptotically linear relationship. Compared with compartmental analysis of untransformed data, the graphic method enables derivation of regional distribution volumes that are free from assumptions about the underlying compartmental configuration. In this article, we describe statistical bias associated with this nonlinear transformation method. METHODS: Theoretic analysis, Monte Carlo simulation, and statistical analysis of PET data were used to test the graphic method for bias. RESULTS: Mean zero noise is associated with underestimation of distribution volumes when data are analyzed with graphic analysis, whereas this effect does not occur when the same data are analyzed by nonlinear regression and compartmental analysis. Moreover, this effect depends on the magnitude of the distribution volume, so that the bias is more pronounced in regions with high receptor density than regions with low receptor density or no receptors (region of reference). CONCLUSION: These results indicate that conventional kinetic analysis of untransformed data is less sensitive to mean zero noise than is graphic analysis of nonlinearly transformed data.     

Non-specific binding of transferrin and lactoferrin to polystyrene culture tubes: role of the radioligand

While evaluating the role of iron-binding glycoproteins on the in vitro uptake of 67Ga and 59Fe by tumor cells, it was observed that these radiometals bind to polystyrene culture tubes in the presence of transferrin or lactoferrin. The amount of 67Ga or 59Fe bound to the tube increases with glycoprotein concentrations up to 20 microgram/ml and decreases thereafter. This biphasic response is a reflection of metal-protein affinity and is greatest for 59Fe-lactoferrin. With 125I-labeled transferrin and lactoferrin, the amount of tube-bound radioactivity was inversely dependent on the glycoprotein concentration suggesting that glycoproteins bind to a limited number of binding sites on the tube wall. Approximately 10(13) glycoprotein molecules were bound per tube with an affinity constant of 1.89 X 10(7) 1/M for transferrin and 1.08 X 10(7) l/M for lactoferrin. These sites are not specific since addition of albumin inhibited the binding of radiolabeled glycoproteins to the tube. In the light of these observations, caution is required in interpreting results of cell culture experiments which have not directed attention to protein-plastic interaction.     

Where have we got to with neuroreceptor mapping of the human brain?

In the past two decades, tritiated radioligand receptor binding, a tool commonly used to investigate the site of action of drugs in laboratory animals, has provided a vast body of information on neuropharmacology and neurobiology. Several neurological and psychiatric diseases have been related to neurotransmitter and receptor disorders. In order to study ligand interactions with receptors in vivo in humans, new tracers capable of carrying a gamma-emitting radionuclide to the receptor have been designed. Emission computerized tomography (ECT) techniques such as positron (PET) or single photon emission tomography (SPET) allow monitoring of the time-course of regional tissue concentration of these radiolabelled ligands. PET and SPET each have their inherent advantages and drawbacks. The cyclotron-based technology of PET is a demanding and expensive technique that, to date, is still mainly reserved for research purposes. It is hoped that once the scientific basis of a physiopathological study is established using PET, diagnostic information might be provided by the more readily available SPET technology. The purpose of this article is to review the current state of receptor-binding gamma-emitting radioligands and to present the clinical potential of these new kinds of radiopharmaceuticals in clinical investigation.     

Evaluation of non-uniform weighting in non-linear regression for pharmacokinetic neuroreceptor modelling

OBJECTIVE: Pharmacokinetic modelling of dynamic PET data has become an important tool to analyse in-vivo studies in humans and animals. Estimation of the model parameters often requires non-linear regression of an objective function such as weighted least squares. Since the noise properties of the data are not known exactly in practice, several weighting schemes have been proposed. The objective of this study was to evaluate the impact of commonly used weights on neuroreceptor quantification with the simplified reference tissue model (SRTM). METHODS: We compared the following weights: uniform, Poisson statistics-based ideal and noisy weights, iterative weighting, and a noise-free approximation of Poisson weights. Ten thousand time-activity curves (TACs) were simulated for several noise levels and the three neuroreceptor PET ligands C-(+)McN5652, C-DASB, and C-raclopride. Each TAC was fitted using weighted non-linear regression of the SRTM. We assessed bias and variation of the parameter estimates as well as quality of fit and parameter distributions. RESULTS: Results differed substantially between ligands and between model parameters. Best parameter estimates were obtained with the noise-free approximation of Poisson weights. The often-used noisy Poisson weights performed worst for all ligands. Uniform weighting gave acceptable parameter estimates for most setups. CONCLUSION: 'Choice of weights' is important in pharmacokinetic neuroreceptor quantification with the SRTM. Weights estimated directly from noisy data should be avoided as they can severely degrade parameter estimation and the statistical power of a study. If the noise characteristic of the data is unknown, uniform weighting is recommended.     

Models and methods for derivation of in vivo neuroreceptor parameters with PET and SPECT reversible radiotracers

The science of quantitative analysis of PET and SPECT neuroreceptor imaging studies has grown considerably over the past decade. A number of methods have been proposed in which receptor parameter estimation results from fitting data to a model of the underlying kinetics of ligand uptake in the brain. These approaches have come to be collectively known as model-based methods and several have received widespread use. Here, we briefly review the most frequently used methods and examine their strengths and weaknesses. Kinetic modeling is the most direct implementation of the compartment models, but with some tracers accurate input function measurement and good compartment configuration identification can be difficult to obtain. Other methods were designed to overcome some particular vulnerability to error of classical kinetic modeling, but introduced new vulnerabilities in the process. Reference region methods obviate the need for arterial plasma measurement, but are not as robust to violations of the underlying modeling assumptions as methods using the arterial input function. Graphical methods give estimates of V(T) without the requirement of compartment model specification, but provide a biased estimator in the presence of statistical noise. True equilibrium methods are quite robust, but their use is limited to experiments with tracers that are suitable for constant infusion. In conclusion, there is no universally "best" method that is applicable to all neuroreceptor imaging studies, and carefully evaluation of model-based methods is required for each radiotracer.     

Research applications of selected 123I-labeled neuroreceptor SPECT imaging ligands

Neuroreceptor imaging is a promising area of brain imaging used to investigate various neurodegenerative and neuropsychiatric disorders. In the research setting, radiopharmaceuticals targeted to specific areas of the brain are used along with SPECT to assess and analyze functional mechanisms within brain structures. Utilization of the data that are collected from these studies may aid in the development of drug therapies that can be used to relieve symptoms or delay progression of certain disease states. After reading this article, the nuclear medicine technologist should be able to identify and describe the role of structures in the brain, identify various radiopharmaceuticals used and the disease states they demonstrate, and become familiar with the drugs used in treatment of these disorders.     

Labeling of hematoporphyrin with 105Rh and binding studies with human gamma globulin

Labeling of hematoporphyrin with ¹⁰⁵Rh at stoichiometric concentrations is described. Labeling efficiencies of up to 93% could be obtained at pH 9.0 in bicarbonate buffer. Solvent extraction of ¹⁰⁵Rhhematoporphyrin into methyl isobutyl ketone was used to estimate the complex yield. The complex showed high stability and no loss of ¹⁰⁵Rh was seen throughout the 6 days of study. ¹⁰⁵Rhhematoporphyrin when incubated with human γ globulin was seen to be quantitatively bound to the protein. This procedure may be used for labeling monoclonal antibodies with ¹⁰⁵Rh for therapeutic applications.     

Effects of point spread function-based image reconstruction on neuroreceptor binding in positron emission tomography study with [<Superscript>11</Superscript>C]FLB 457

The ordered subset expectation maximization with a point spread function (OSEM-PSF) was developed to improve the spatial resolution of reconstructed positron emission tomography (PET) images and has been reported to improve the contrast of hot spots in PET studies for oncology. However, in neuroreceptor imaging, the regional radioactivity concentration changes dynamically during the scan, and the effects of the PSF may differ among various radioligands or quantification methods. In this study, we investigated the effects of the PSF on quantification in PET studies with [¹¹C]FLB 457 of dopamine D₂ receptors, using both phantom and human data acquired by the Siemens Biograph 16 imaging platform. In the phantom studies, we evaluated the hot contrast recovery coefficient (HCRC) for variously sized hot spheres and the linearity between the measured and true radioactivities in OSEM-PSF images. Next, in the human studies with [¹¹C]FLB 457, radioactivity concentrations and binding potentials for the cerebral cortex and thalamus were compared between images reconstructed with and without PSF. In the phantom studies, the OSEM-PSF images showed a better HCRC compared to images without PSF, and they showed a good linear correlation with true radioactivity. In the human studies, the radioactivity concentration increased especially in small regions with high accumulation of [¹¹C]FLB 457 when the PSF was included. However, little difference in the binding potentials was observed for the target regions between both types of reconstructed images. In conclusion, PSF-based reconstruction reduced the spill-over phenomena in small hot regions; however, it caused no increase in the binding potentials in the [¹¹C]FLB 457 studies.     

A novel radioligand for glycine transporter 1: characterization and use in autoradiographic and in vivo brain occupancy studies

INTRODUCTION: In an effort to develop agents to test the NMDA hypofunction hypothesis of schizophrenia, benchmark compounds from a program to discover potent, selective, competitive glycine transporter 1 (GlyT1) inhibitors were radiolabeled in order to further study the detailed pharmacology of these inhibitors and the distribution of GlyT1 in brain. We here report the in vitro characterization of [35S](S)-2-amino-4-chloro-N-(1-(4-phenyl-1-(propylsulfonyl)piperidin-4-yl)ethyl)benzamide ([35S]ACPPB), a radiotracer developed from a potent and selective non-sarcosine-derived GlyT1 inhibitor, its use in autoradiographic studies to localize (S)-2-amino-6-chloro-N-(1-(4-phenyl-1-(propylsulfonyl)piperidin-4-yl)ethyl)benzamide (ACPPB) binding sites in rat and rhesus brain and for in vivo occupancy assays of competitive GlyT1 inhibitors. METHODS: Functional potencies of unlabeled compounds were characterized by [14C]glycine uptake into JAR (human placental choriocarcinoma) cells and synaptosomes. Radioligand binding studies were performed with tissue homogenates. Autoradiographic studies were performed on tissue slices. RESULTS: ACPPB is a potent (Kd=1.9 nM), selective, GlyT1 inhibitor that, when radiolabeled with [35S], is a well-behaved radioligand with low nondisplaceable binding. Autoradiographic studies of rat and rhesus brain slices with this ligand showed that specific binding sites were plentiful and nonhomogeneously distributed, with high levels of binding in the brainstem, cerebellar white matter, thalamus, cortical white matter and spinal cord gray matter. In vivo studies demonstrate displaceable binding of [35S]ACPPB in rat brain tissues following iv administration of this radioligand. CONCLUSIONS: This is the first report of detailed anatomical localization of GlyT1 using direct radioligand binding, and the first demonstration that an in vivo occupancy assay is feasible, suggesting that it may also be feasible to develop positron emission tomography tracers for GlyT1.     

SPECT脑血流灌注显像和受体显像对抑郁症的研究

抑郁症的影像学检查有结构性脑成像和功能性脑成像,CT和MRI检查脑结构的异常既不显著又不具有特异性,而PET和SPECT提供了在活体内对抑郁症进行局部脑血流灌注和中枢神经递质与受体结构和功能的研究,为进一步探讨抑郁症的病因、发病机制以及临床治疗提供客观依据.     

Specific in vivo binding to the norepinephrine transporter demonstrated with the PET radioligand, (S,S)-[11C]MeNER

(S,S)-2-(alpha-(2-Methoxyphenoxy)benzyl)morpholine (MeNER), an O-methyl analog of the selective and potent norepinephrine transporter (NET) inhibitor, (S,S)-reboxetine, and its less active enantiomer, (R,R)-MeNER, have each been radiolabeled by O-methylation of their corresponding phenolic precursors in good yields from [(11)C]methyl iodide or [(11)C]methyl triflate. Radiochemical purities were >99% and specific radioactivity at time of injection was about 74 GBq/micromol. Autoradiographic examination of (S,S)-[(11)C]MeNER binding to human brain slices post mortem indicated specific binding in a brain region including the locus coeruleus. PET examination of both [(11)C]MeNER enantiomers in a cynomolgus monkey demonstrated a higher specific binding of the (S,S)-enantiomer with ratios of 1.4-1.6 in the lower brainstem, mesencephalon and thalamus to striatum. Pretreatment with the NET ligand, desipramine, decreased the specific binding of (S,S)-[(11)C]MeNER. Labeled metabolites of [(11)C]MeNER were all more polar. (S,S)-[(11)C]MeNER is a good lead compound in the search for a selective radioligand for quantitation of NET in the human brain in vivo.     

Traditional receptor theory and its application to neuroreceptor measurements in functional imaging

The mathematical, biological discipline of receptor pharmacology and the graphical methods of describing receptor behaviour evolved over a century of painstaking observation and model testing. Laws regarding in vitro theories are classically considered inoperative in vivo. Nevertheless, functional imaging techniques have rapidly evolved to allow receptor measurement and rules of thumb have been developed which clearly prove valid receptor parameters can be derived from functional imaging studies. The field is evolving so rapidly now that nuclear medicine researchers are in danger in applying these techniques without recourse to an understanding of the orthodox discipline of receptor pharmacology. This review attempts to document the basis of receptor pharmacology and to give an account of the theoretical and practical basis on which this can be applied in vivo. The review is targeted towards single-photon emission tomography because of the rapid growth in the area, but many parts draw on the literature relating to positron emission tomography since the first translation of in vitro to in vivo measurement was performed with this technique.     

Assessing neuroreceptor occupancy by continuous infusion of carbon-11 labeled radioligands

In carbon-11 labeled radiotracer studies of neuroreceptors in the brain, a state of practical equilibrium, monitored with a simple dual-probe detector system, was achieved by injection of a bolus of tracer followed by a continuous infusion of the tracer. After investigating several different bolus-to-infusion activity ratios, we observed that a practical equilibrium state could be achieved with a slope of 0.02±0.001 cpsxkg/µCi/min for [¹¹C]carfentanil studies in normal human beings. The usefulness of this approach to assess neuroreceptor occupancy was demonstrated in two cases.     

Imaging of the muscarinic acetylcholine neuroreceptor in rats with the M2 selective agonist [18F]FP-TZTP

Introduction[¹⁸F]FP-TZTP is an M2 muscarinic subtype selective receptor-binding radiotracer used in vivo to image human and nonhuman primate brain following both bolus injection and infusion. In order to carry out repeated studies in rodents, the techniques developed for primates must be transferred to rodents with the same precision. This includes obtaining a metabolite-corrected input function.MethodsWe compared bolus injection with constant infusion in rats that were awake or under isoflurane anesthesia. Brain–plasma and brain–blood distribution ratios were calculated by dividing brain ¹⁸F concentrations, determined in vivo by positron emission tomography imaging with the Advanced Technology Laboratory Animal Scanner, ex vivo by direct counting in excised brain tissue or by quantitative autoradiography by the plasma or whole blood concentrations that had been corrected for metabolite contents.ResultsBlood volume constraints prevented adequate blood sampling to define a precise input function after bolus injection, thus preventing full kinetic analysis. Constant infusion, however, required fewer blood samples to define the input function, allowing calculation of distribution ratios, but complete equilibrium between plasma and tissue had not yet been reached after 120 min.ConclusionOur results showed that the blood clearance and metabolism were too rapid to obtain a reproducible input function after bolus injection. The equilibrium distribution ratios did not lead to precise biochemical parameters, but the constant infusion was more suitable in that distribution ratios between tissue and plasma were statistically more precise. Constant infusion is the better approach for studying [¹⁸F]FP-TZTP by small animal imaging.     

A strategy for increasing the brain uptake of a radioligand in animals: use of a drug that inhibits plasma protein binding

A positron-emitter labeled radioligand for the glycine-binding site of the N-methyl-D-aspartate (NMDA) receptor, [(11)C]L-703,717, was examined for its ability to penetrate the brain in animals by simultaneous use with drugs having high-affinity separate binding sites on human serum albumin. [(11)C]L-703,717 has poor blood-brain barrier (BBB) permeability because it binds tightly to plasma proteins. Co-injection of warfarin (50-200 mg/kg), a drug that binds to albumin and resembles L-703,717 in structure, dose-dependently enhanced the penetration by [(11)C]L-703,717 in mice, resulting in a five-fold increase in the brain radioactivity at 1 min after the injection. Drugs structurally unrelated to L-703,717, salicylate, phenol red, and L-tryptophan, were less effective or ineffective in increasing the uptake of [(11)C]L-703,717. These results suggest that the simultaneous use of a drug that inhibits the binding of a radioligand to plasma proteins is a useful way to overcome the poor BBB permeability of the radioligand triggered by its tight binding to plasma proteins. In brain distribution studies in rodents, it was found that, after the increase in brain uptake with warfarin, much of the glycine site antagonist accumulates in the cerebellum but its pharmacological specificity did not match the glycine site of NMDA receptors.