PET radiotracers: crossing the blood-brain barrier and surviving metabolism

Discovery of Phosphodiesterase 10A (PDE10A) PET Tracer AMG 580 to Support Clinical Studies

We report the discovery of PDE10A PET tracer AMG 580 developed to support proof of concept studies with PDE10A inhibitors in the clinic. To find a tracer with higher binding potential (BPND) in NHP than our previously reported tracer 1, we implemented a surface plasmon resonance assay to measure the binding off-rate to identify candidates with slower washout rate in vivo. Five candidates (2-6) from two structurally distinct scaffolds were identified that possessed both the in vitro characteristics that would favor central penetration and the structural features necessary for PET isotope radiolabeling. Two cinnolines (2, 3) and one keto-benzimidazole (5) exhibited PDE10A target specificity and brain uptake comparable to or better than 1 in the in vivo LC-MS/MS kinetics distribution study in SD rats. In NHP PET imaging study, [(18)F]-5 produced a significantly improved BPND of 3.1 and was nominated as PDE10A PET tracer clinical candidate for further studies.

Strategies towards in vivo imaging of active transglutaminase type 2 using positron emission tomography

Transglutaminase type 2 (TG2) is increasingly linked to the pathogenesis of several diseases, such as celiac disease, cancer, and fibrotic and neurodegenerative diseases. In parallel with becoming an attractive target for therapy, interest in the development of compounds for in vivo imaging of TG2 is rising. Such imaging biomarkers might assist in clarifying the role of TG2 in pathology and in monitoring TG2 inhibition in vivo and thus assist in drug development. In this review, the latest results together with various strategies in TG2 PET tracer development are discussed, including radiolabelling of irreversible and reversible active-site inhibitors, as well as allosteric inhibitors, acyl-donor and acyl-acceptor substrates, and anti-TG2 monoclonal antibodies.

Radiosynthesis and in vivo Evaluation of Carbon-11 (2S)-3-(1H-Indol-3-yl)-2-{[(4-methoxyphenyl)carbamoyl]amino}-N-{[1-(5-methoxypyridin-2-yl)cyclohexyl]methyl}propanamide: An Attempt to Visualize Brain Formyl Peptide Receptors in Mouse Models of Neuroinflammation

Here, we describe the very first attempt to visualize in vivo formyl peptide receptors (FPRs) in mouse brain by positron emission tomography (PET). FPRs are expressed in microglial cells where they mediate chemotactic activity of β-amyloid peptide in Alzheimer disease and, thus, are involved in neuroinflammatory processes. To this purpose, we have selected (2S)-3-(1H-Indol-3-yl)-2-{[(4-methoxyphenyl)carbamoyl]amino}-N-{[1-(5-methoxypyridin-2-yl)cyclohexyl]methyl}propanamide ((S)-1), that we have previously identified as a potent non-peptidic FPR agonist. (S)-[(11) C]-1 has been prepared in high radiochemical yield. (S)-[(11) C]-1 showed very low penetration of blood-brain barrier and, thus, was unable to accumulate into the brain. In addition, (S)-[(11) C]-1 was not able to label FPRs receptors in brain slices of PS19 and APP23 mice, two animal models of Alzheimer disease. Although (S)-[(11) C]-1 was not suitable to visualize FPRs in the brain, this study provides useful information for the design and characterization of future potential PET radioligands for visualization of brain FPRs by PET.

Synthesis and Preclinical Evaluation of Sulfonamido-based [(11)C-Carbonyl]-Carbamates and Ureas for Imaging Monoacylglycerol Lipase

Monoacylglycerol lipase (MAGL) is a 33 kDa member of the serine hydrolase superfamily that preferentially degrades 2-arachidonoylglycerol (2-AG) to arachidonic acid in the endocannabinoid system. Inhibition of MAGL is not only of interest for probing the cannabinoid pathway but also as a therapeutic and diagnostic target for neuroinflammation. Limited attempts have been made to image MAGL in vivo and a suitable PET ligand for this target has yet to be identified and is urgently sought to guide small molecule drug development in this pathway. Herein we synthesized and evaluated the physiochemical properties of an array of eleven sulfonamido-based carbamates and ureas with a series of terminal aryl moieties, linkers and leaving groups. The most potent compounds were a novel MAGL inhibitor, N-((1-(1H-1,2,4-triazole-1-carbonyl)piperidin-4-yl) methyl)-4-chlorobenzenesulfonamide (TZPU; IC₅₀ = 35.9 nM), and the known inhibitor 1,1,1,3,3,3-hexafluoropropan-2-yl 4-(((4-chlorophenyl)sulfonamido) methyl)piperidine-1-carboxylate (SAR127303; IC₅₀ = 39.3 nM), which were also shown to be selective for MAGL over fatty acid amide hydrolase (FAAH), and cannabinoid receptors (CB1 & CB2). Both of these compounds were radiolabeled with carbon-11 via [¹¹C]COCl₂, followed by comprehensive ex vivo biodistribution and in vivo PET imaging studies in normal rats to determine their brain permeability, specificity, clearance and metabolism. Whereas TZPU did not show adequate specificity to warrant further evaluation, [¹¹C]SAR127303 was advanced for preliminary PET neuroimaging studies in nonhuman primate. The tracer showed good brain permeability (ca. 1 SUV) and heterogeneous regional brain distribution which is consistent with the distribution of MAGL.

PET tracers for imaging brain α7 nicotinic receptors: an update

Positron emission tomography (PET) molecular imaging of brain targets is a powerful tool to diagnose, follow up, and develop treatments and personalized medicine for a number of acute and chronic brain disorders. The availability of β+ emitter tracers labelled with [(11)C] or [(18)F] having optimal characteristics of affinity and selectivity for alpha-7 nicotinic receptors (α7R) has received considerable attention, due to the major implication of these receptors in brain functions. The aim of this review is to identify the interest and need for the in vivo exploration of α7R by PET molecular imaging, which tools are currently available for this and how to progress.

Comparative assessment of (18) F-Mefway as a serotonin 5-HT1A receptor PET imaging agent across species: Rodents, nonhuman primates, and humans

We have developed (18) F-trans-Mefway ((18) F-Mefway) for positron emission tomography (PET) imaging studies of serotonin 5-HT1A receptors which are implicated in various brain functions. Translation of imaging the 5-HT1A receptor in animal models to humans will facilitate an understanding of the role of the receptor in human brain disorders. We report comparative brain distribution of (18) F-Mefway in normal mice, rats, monkeys, and healthy human volunteers. Mefway was found to be very selective, with subnanomolar affinity for the 5-HT1A receptor. Affinities of >55 nM were found for all other human-cloned receptor subtypes tested. Mefway was found to be a poor substrate (>30 μM) for the multidrug resistance 1 protein, suggesting low likelihood of brain uptake being affected by P-glycoprotein. Cerebellum was used as a reference region in all imaging studies across all species due to the low levels of (18) F-Mefway binding. Consistent binding of (18) F-Mefway in cortical regions, hippocampus, and raphe was observed across all species. (18) F-Mefway in the human brain regions correlated with the known postmortem distribution of 5-HT1A receptors. Quantitation of raphe was affected by the resolution of the PET scanners in rodents, whereas monkeys and humans showed a raphe to cerebellum ratio of approximately 3. (18) F-Mefway appears to be an effective 5-HT1A receptor imaging agent in all models, including humans. (18) F-Mefway therefore may be used to quantify 5-HT1A receptor distribution in brain regions for the study of various CNS disorders. J. Comp. Neurol. 524:1457-1471, 2016. © 2015 Wiley Periodicals, Inc.

Synthesis, radiolabeling and evaluation of novel amine guanidine derivatives as potential positron emission tomography tracers for the ion channel of the N-methyl-d-aspartate receptor

The N-Methyl-d-Aspartate receptor (NMDAR) is involved in many neurological and psychiatric disorders including Alzheimer's disease and schizophrenia. The aim of this study was to develop a positron emission tomography (PET) ligand to assess the bio-availability of the NMDAR ion channel in vivo. A series of tri-N-substituted diarylguanidines was synthesized and their in vitro binding affinities for the NMDAR ion channel assessed in rat forebrain membrane fractions. Compounds 21, 23 and 26 were radiolabeled with either carbon-11 or fluorine-18 and ex vivo biodistribution and metabolite studies were performed in Wistar rats. Biodistribution studies showed high uptake especially in prefrontal cortex and lowest uptake in cerebellum. Pre-treatment with MK-801, however, did not decrease uptake of the radiolabeled ligands. In addition, all three ligands showed fast metabolism.

Synthesis and evaluation of 6-(3-[(18)F]fluoro-2-hydroxypropyl)-substituted 2-pyridylbenzothiophenes and 2-pyridylbenzothiazoles as potential PET tracers for imaging Aβ plaques

3-[(18)F]Fluoro-2-hydroxypropyl substituted compounds were synthesized and evaluated as novel (18)F-labeled PET tracers for imaging Aβ plaque in a living brain. All compounds exhibited high binding affinities toward the synthetic Aβ1-42 aggregate and/or Alzheimer's disease brain homogenate. In the microPET study with normal mice, the 3-[(18)F]fluoro-2-hydroxypropyl substituted compounds resulted in fast brain washout by reducing the lipophilicities of the compounds. Intriguingly, (S)-configured PET tracers, (S)-[(18)F]1b and (S)-[(18)F]1c, exhibited a 2.8 and 4.0-fold faster brain washout rate at a peak/30 min in the mouse brain than the corresponding (R)-configured PET tracers despite there being no meaningful difference in binding affinities toward Aβ plaque. A further evaluation of (S)-[(18)F]1c with healthy rhesus monkeys also revealed excellent clearance from the frontal cortex with ratios of 7.0, 16.0, 30.0 and 49.0 at a peak/30, 60, 90, and 120 min, respectively. These results suggest that (S)-[(18)F]1c may be a potential PET tracer for imaging Aβ plaque in a living brain.

Toward PET imaging of A2B adenosine receptors: a carbon-11 labeled triazinobenzimidazole tracer: Synthesis and imaging of a new A2B PET tracer

INTRODUCTION

A2B adenosine receptors (ARs) are commonly defined as "danger" sensors because they are triggered during cell injury when the endogenous molecule, adenosine, increases rapidly. These receptors, together with the other receptor subtypes (A1, A2A and A3), exert a wide variety of immunomodulating and (cyto)protective effects, thus representing a pivotal therapeutic target for different pathologies including diabetes, tumors, cardiovascular diseases, pulmonary fibrosis and others. The limited availability of potent and selective ligands for A2B ARs has prevented this receptor to emerge both as therapeutic and diagnostic target.

METHODS

Recently, a new class of potent A2B ARs antagonists was developed featuring the triazinobenzimidazole scaffold. Starting from this chemotype, we synthesized a new radiotracer, [(11)C]-4 (1-[(11)C]methyl-3-phenyl triazino[4,3-a]benzimidazol-4(1H)-one), and investigated the pharmacokinetics of this compound in vivo to define its potential use in the imaging of A2B AR with positron emission tomography.

RESULTS

[(11)C]-4 showed a very high chemical and blood stability. Results of in vivo and ex vivo experiments underlined the ability of this molecule to bind the A2B AR and correlated with the A2B AR protein and gene expression data.

CONCLUSIONS

Although further studies are necessary, these data suggest that [(11)C]-4 may represent a good lead compound for the development of novel selective and potent A2B AR radiotracers, and a new option for the clinical investigation of several pathophysiological processes and chronic diseases.

Development of TASP0410457 (TASP457), a novel dihydroquinolinone derivative as a PET radioligand for central histamine H3 receptors

Background

Histamine H₃ receptor (H₃R) is a potential therapeutic target of sleep- and cognition-related disorders. The purpose of the present study is to develop a novel positron emission tomography (PET) ligand for H₃Rs from dihydroquinolinone derivatives, which we previously found to have high affinity with these receptors.

Methods

Six compounds were selected from a dihydroquinolinone compound library based on structural capability for ¹¹C labeling and binding affinity for H₃Rs. Their in vivo kinetics in the rat brain were examined in a comparative manner by liquid chromatography and tandem mass spectrometry (LC-MS/MS). Chemicals with appropriate kinetic properties were then labeled with ¹¹C and evaluated in rats and monkeys using PET.

Results

Of the six compounds, TASP0410457 (also diminutively called TASP457) and TASP0434988 exhibited fast kinetics and relatively high brain uptakes in ex vivo LC-MS/MS and were selected as candidate PET imaging agents. PET data in rat brains were mostly consistent with LC-MS/MS findings, and rat and monkey PET scans demonstrated that [¹¹C]TASP0410457 was superior to [¹¹C]TASP0434988 for high-contrast H₃R PET imaging. In the monkey brain PET, distribution volume for [¹¹C]TASP0410457 could be quantified, and receptor occupancy by a nonradioactive compound was measurable using this radioligand. The specific binding of [¹¹C]TASP0410457 to H₃Rs was confirmed by autoradiography using rat and monkey brain sections.

Conclusions

We developed [¹¹C]TASP0410457 as a radioligand enabling a robust quantification of H₃Rs in all brain regions and demonstrated the utility of ex vivo LC-MS/MS and in vivo PET assays for selecting appropriate imaging tracers. [¹¹C]TASP0410457 will help to examine the implication of H₃Rs in neuropsychiatric disorders and to characterize emerging therapeutic agents targeting H₃Rs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0170-2) contains supplementary material, which is available to authorized users.

Measuring specific receptor binding of a PET radioligand in human brain without pharmacological blockade: The genomic plot

PET studies allow in vivo imaging of the density of brain receptor species. The PET signal, however, is the sum of the fraction of radioligand that is specifically bound to the target receptor and the non-displaceable fraction (i.e. the non-specifically bound radioligand plus the free ligand in tissue). Therefore, measuring the non-displaceable fraction, which is generally assumed to be constant across the brain, is a necessary step to obtain regional estimates of the specific fractions. The nondisplaceable binding can be directly measured if a reference region, i.e. a region devoid of any specific binding, is available. Many receptors are however widely expressed across the brain, and a true reference region is rarely available. In these cases, the nonspecific binding can be obtained after competitive pharmacological blockade, which is often contraindicated in humans. In this work we introduce the genomic plot for estimating the nondisplaceable fraction using baseline scans only. The genomic plot is a transformation of the Lassen graphical method in which the brain maps of mRNA transcripts of the target receptor obtained from the Allen brain atlas are used as a surrogate measure of the specific binding. Thus, the genomic plot allows the calculation of the specific and nondisplaceable components of radioligand uptake without the need of pharmacological blockade. We first assessed the statistical properties of the method with computer simulations. Then we sought ground-truth validation using human PET datasets of seven different neuroreceptor radioligands, where nonspecific fractions were either obtained separately using drug displacement or available from a true reference region. The population nondisplaceable fractions estimated by the genomic plot were very close to those measured by actual human blocking studies (mean relative difference between 2% and 7%). However, these estimates were valid only when mRNA expressions were predictive of protein levels (i.e. there were no significant post-transcriptional changes). This condition can be readily established a priori by assessing the correlation between PET and mRNA expression.

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.

Comparison of New Tau PET-Tracer Candidates With [18F]T808 and [18F]T807

Early clinical results of two tau tracers, [(18)F]T808 and [(18)F]T807, have recently been reported. In the present study, the biodistribution, radiometabolite quantification, and competition-binding studies were performed in order to acquire comparative preclinical data as well as to establish the value of T808 and T807 as benchmark compounds for assessment of binding affinities of eight new/other tau tracers. Biodistribution studies in mice showed high brain uptake and fast washout.In vivoradiometabolite analysis using high-performance liquid chromatography showed the presence of polar radiometabolites in plasma and brain. No specific binding of [(18)F]T808 was found in transgenic mice expressing mutant human P301L tau. In semiquantitative autoradiography studies on human Alzheimer disease slices, we observed more than 50% tau selective blocking of [(18)F]T808 in the presence of 1 µmol/L of the novel ligands. This study provides a straightforward comparison of the binding affinity and selectivity for tau of the reported radiolabeled tracers BF-158, BF-170, THK5105, lansoprazole, astemizole, and novel tau positron emission tomography ligands against T807 and T808. Therefore, these data are helpful to identify structural requirements for selective interaction with tau and to compare the performance of new highly selective and specific radiolabeled tau tracers.

Synthesis and Preliminary PET Imaging Studies of a FAAH Radiotracer ([¹¹C]MPPO) Based on α-Ketoheterocyclic Scaffold

Fatty acid amide hydrolase (FAAH) is one of the principle enzymes for metabolizing endogenous cannabinoid neurotransmitters such as anandamide, and thus regulates endocannabinoid (eCB) signaling. Selective pharmacological blockade of FAAH has emerged as a potential therapy to discern the endogenous functions of anandamide-mediated eCB pathways in anxiety, pain, and addiction. Quantification of FAAH in the living brain by positron emission tomography (PET) would help our understanding of the endocannabinoid system in these conditions. While most FAAH radiotracers operate by an irreversible ("suicide") binding mechanism, a FAAH tracer with reversibility would facilitate quantitative analysis. We have identified and radiolabeled a reversible FAAH inhibitor, 7-(2-[(11)C]methoxyphenyl)-1-(5-(pyridin-2-yl)oxazol-2-yl)heptan-1-one ([(11)C]MPPO) in 13% radiochemical yield (nondecay corrected) with >99% radiochemical purity and 2 Ci/μmol (74 GBq/μmol) specific activity. The tracer showed moderate brain uptake (0.8 SUV) with heterogeneous brain distribution. However, blocking studies with a potent FAAH inhibitor URB597 demonstrated a low to modest specificity to the target. Measurement of lipophilicity, metabolite, and efflux pathway analysis were also performed to study the pharmacokinetic profile of [(11)C]MPPO. In all, we reported an efficient radiolabeling and preliminary evaluation of the first-in-class FAAH inhibitor [(11)C]MPPO with α-ketoheterocyclic scaffold.

Blood-brain and retinal barriers show dissimilar ABC transporter impacts and concealed effect of P-glycoprotein on a novel verapamil influx carrier

BACKGROUND AND PURPOSE

The respective impact and interplay between ABC (P-glycoprotein/P-gp/Abcb1a, BCRP/ABCG2, MRP/ABCC) and SLC transporter functions at the blood-brain barrier (BBB) and blood-retinal barriers (BRB) are incompletely understood.

EXPERIMENTAL APPROACH

We measured the initial cerebral and retinal distribution of selected ABC substrates by in situ carotid perfusion using P-gp/Bcrp knockout mice and chemical ABC/SLC modulation strategies. P-gp, Bcrp, Mrp1 and Mrp4 were studied by confocal retina imaging.

KEY RESULTS

Chemical or physical disruption of P-gp increased [(3) H]-verapamil transport by ~10-fold at the BBB and ~1.5-fold at the BRB. [(3) H]-Verapamil transport involved influx-mediated by an organic cation clonidine-sensitive/diphenhydramine-sensitive proton antiporter at both barriers; this effect was unmasked when P-gp was partially or fully inhibited/disrupted at the BBB. Studies of [(3) H]-mitoxantrone and [(3) H]-zidovudine transport suggested, respectively, that Bcrp efflux was less involved at the BRB than BBB, whereas Mrps were significantly and similarly involved at both barriers. Confocal imaging showed that P-gp and Bcrp were expressed in intra-retinal vessels (inner BRB/iBRB) but absent from the blood/basal membrane of cells of the retinal pigment epithelium (outer BRB/oBRB/RPE) where, in contrast, Mrp1 and Mrp4 were localized.

CONCLUSIONS AND IMPLICATIONS

P-gp, Bcrp, Mrp1 and Mrp4 are differentially expressed at the outer and inner BRB, resulting in an altered ability to limit substrate distribution at the retina as compared with the BBB. [(3) H]-Verapamil distribution is not P-gp-specific and involves a proton antiporter at both the BBB and BRB. However, this transport is concealed by P-gp at the BBB, but not at the BRB, where P-gp activity is reduced.

Insight into the Molecular Imaging of Alzheimer's Disease

Alzheimer's disease is a complex neurodegenerative disease affecting millions of individuals worldwide. Earlier it was diagnosed only via clinical assessments and confirmed by postmortem brain histopathology. The development of validated biomarkers for Alzheimer's disease has given impetus to improve diagnostics and accelerate the development of new therapies. Functional imaging like positron emission tomography (PET), single photon emission computed tomography (SPECT), functional magnetic resonance imaging (fMRI), and proton magnetic resonance spectroscopy provides a means of detecting and characterising the regional changes in brain blood flow, metabolism, and receptor binding sites that are associated with Alzheimer's disease. Multimodal neuroimaging techniques have indicated changes in brain structure and metabolic activity, and an array of neurochemical variations that are associated with neurodegenerative diseases. Radiotracer-based PET and SPECT potentially provide sensitive, accurate methods for the early detection of disease. This paper presents a review of neuroimaging modalities like PET, SPECT, and selected imaging biomarkers/tracers used for the early diagnosis of AD. Neuroimaging with such biomarkers and tracers could achieve a much higher diagnostic accuracy for AD and related disorders in the future.

Synthesis and pre-clinical evaluation of a [18F]fluoromethyl-tanaproget derivative for imaging of progesterone receptor expression

This work features the synthesis and biological evaluation of a novel fluorinated derivative of tanaproget and a carbon-11 labelled analogue, a high affinity non-steroidal PR ligand, as a possible candidate for imaging PR expressionin vivo.

Blood brain barrier: An overview on strategies in drug delivery, realistic in vitro modeling and in vivo live tracking

Blood brain barrier (BBB) is a group of astrocytes, neurons and endothelial cells, which makes restricted passage of various biological or chemical entities to the brain tissue. It gives protection to brain at one hand, but at the other hand it has very selective permeability for bio-actives and other foreign materials and is one of the major challenges for the drug delivery. Nanocarriers are promising to cross BBB utilizing alternative route of administration such as intranasal and intra-carotid drug delivery which bypasses BBB. In future more optimized drug delivery system can be achieved by compiling the best routes with the best carriers. Single photon emission tomography (SPECT) and different brain-on-a-chip in vitro models are being very reliable to study live in vivo tracking of BBB and its pathophysiology, respectively. In the current review we have tried to exploit mechanistically all these to understand and manage the various BBB disruptions in diseased condition along with crossing the hurdles occurring in drug or gene delivery across BBB.

Considerations in the Development of Reversibly Binding PET Radioligands for Brain Imaging

The development of reversibly binding radioligands for imaging brain proteins in vivo, such as enzymes, neurotransmitter transporters, receptors and ion channels, with positron emission tomography (PET) is keenly sought for biomedical studies of neuropsychiatric disorders and for drug discovery and development, but is recognized as being highly challenging at the medicinal chemistry level. This article aims to compile and discuss the main considerations to be taken into account by chemists embarking on programs of radioligand development for PET imaging of brain protein targets.

In vivo quantification of the [(11)C]DASB binding in the normal canine brain using positron emission tomography

Background

[¹¹C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile ([¹¹C]DASB) is currently the mostly used radiotracer for positron emission tomography (PET) quantitative studies of the serotonin transporter (SERT) in the human brain but has never been validated in dogs. The first objective was therefore to evaluate normal [¹¹C]DASB distribution in different brain regions of healthy dogs using PET. The second objective was to provide less invasive and more convenient alternative methods to the arterial sampling-based kinetic analysis.

Results

A dynamic acquisition of the brain was performed during 90 min. The PET images were coregistered with the magnetic resonance images taken prior to the study in order to manually drawn 20 regions of interest (ROIs). The highest radioactivity concentration of [¹¹C]DASB was observed in the hypothalamus, raphe nuclei and thalamus and lowest levels in the parietal cortex, occipital cortex and cerebellum.

The regional radioactivity in those 20 ROIs was quantified using the multilinear reference tissue model 2 (MRTM2) and a semi-quantitative method. The values showed least variability between 40 and 60 min and this time interval was set as the optimal time interval for [¹¹C]DASB quantification in the canine brain. The correlation (R²) between the MRTM2 and the semi-quantitative method using the data between 40 and 60 min was 99.3 % (two-tailed p-value < 0.01).

Conclusions

The reference tissue models and semi-quantitative method provide a more convenient alternative to invasive arterial sampling models in the evaluation of the SERT of the normal canine brain. The optimal time interval for static scanning is set at 40 to 60 min after tracer injection.

Recent Advances in the Development and Application of Radiolabeled Kinase Inhibitors for PET Imaging

Over the last 20 years, intensive investigation and multiple clinical successes targeting protein kinases, mostly for cancer treatment, have identified small molecule kinase inhibitors as a prominent therapeutic class. In the course of those investigations, radiolabeled kinase inhibitors for positron emission tomography (PET) imaging have been synthesized and evaluated as diagnostic imaging probes for cancer characterization. Given that inhibitor coverage of the kinome is continuously expanding, in vivo PET imaging will likely find increasing applications for therapy monitoring and receptor density studies both in- and outside of oncological conditions. Early investigated radiolabeled inhibitors, which are mostly based on clinically approved tyrosine kinase inhibitor (TKI) isotopologues, have now entered clinical trials. Novel radioligands for cancer and PET neuroimaging originating from novel but relevant target kinases are currently being explored in preclinical studies. This article reviews the literature involving radiotracer design, radiochemistry approaches, biological tracer evaluation and nuclear imaging results of radiolabeled kinase inhibitors for PET reported between 2010 and mid-2015. Aspects regarding the usefulness of pursuing selective vs. promiscuous inhibitor scaffolds and the inherent challenges associated with intracellular enzyme imaging will be discussed.

N'-3-(Trifluoromethyl)phenyl Derivatives of N-Aryl-N'-methylguanidines as Prospective PET Radioligands for the Open Channel of the N-Methyl-d-aspartate (NMDA) Receptor: Synthesis and Structure-Affinity Relationships

N-Methyl-d-aspartate (NMDA) receptor dysfunction has been linked to several neuropsychiatric disorders, including Alzheimer's disease, epilepsy, drug addiction, and schizophrenia. A radioligand that could be used with PET to image and quantify human brain NMDA receptors in the activated "open channel" state would be useful for research on such disorders and for the development of novel therapies. To date, no radioligands have shown well-validated efficacy for imaging NMDA receptors in human subjects. In order to discover improved radioligands for PET imaging, we explored structure-affinity relationships in N'-3-(trifluoromethyl)phenyl derivatives of N-aryl-N'-methylguanidines, seeking high affinity and moderate lipophilicity, plus necessary amenability for labeling with a positron-emitter, either carbon-11 or fluorine-18. Among a diverse set of 80 prepared N'-3-(trifluoromethyl)phenyl derivatives, four of these compounds (13, 19, 20, and 36) displayed desirable low nanomolar affinity for inhibition of [(3)H](+)-MK801 at the PCP binding site and are of interest for candidate PET radioligand development.

[carbonyl-11C]4-Fluoro-N-methyl-N-(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide ([11C]FIMX) is an effective radioligand for PET imaging of metabotropic glutamate receptor 1 (mGluR1) in monkey brain

INTRODUCTION

Metabotropic glutamate subtype receptor 1 (mGluR1) is implicated in several neuropsychiatric disorders and is a target for drug development. [(18)F]FIMX ([(18)F]4-fluoro--N-methyl-N--(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide) is an effective radioligand for imaging brain mGluR1 with PET. A similarly effective radioligand with a shorter half-life would usefully allow PET studies of mGluR1 at baseline and after pharmacological or other challenge on the same day. Here we describe the preparation of [(11)C]FIMX for evaluation in monkey with PET.

METHODS

[(11)C]FIMX was prepared via Pd-promoted carbonylation of 1-fluoro-4-iodobenzene with [(11)C]carbon monoxide, aminolysis of the [(11)C]acyl-palladium complex with the requisite Boc-protected amine, and deprotection with HCl in THF. PET scans of [(11)C]FIMX injected into a monkey were performed at baseline and after preblock of mGluR1 with measurement of the arterial input function.

RESULTS

The radiosynthesis required 42 min and gave [(11)C]FIMX in about 5% overall decay-corrected radiochemical yield and with a specific activity of about 100 GBq/μmol. PET in rhesus monkey at baseline showed that radioactivity peaked high in receptor-rich cerebellum and much lower in receptor-poor occipital cortex. Radioactivity in cerebellum declined to 32% of peak at 85 min. VT at baseline appeared stable in all brain regions after 60 min. Under mGluR1 pre-blocked condition, radioactivity uptake in all regions declined more rapidly to a low level. Receptor pre-block reduced VT from 13.0 to 1.5 in cerebellum and from 2.9 to 1.4 in occipital cortex.

CONCLUSION

[(11)C]FIMX is an effective radioligand for imaging mGluR1 in monkey with PET.

Positron Emission Tomographic Imaging in Stroke: Cross-Sectional and Follow-Up Assessment of Amyloid in Ischemic Stroke

Cardiovascular risk factors significantly increase the risk of developing Alzheimer disease. A possible mechanism may be via ischemic infarction–driving amyloid deposition. We conducted a study to determine the presence of β-amyloid in infarct, peri-infarct, and hemispheric areas after stroke. We hypothesized that an infarct would trigger β-amyloid deposition, with deposition over time.

The PET Radioligand 18F-FIMX Images and Quantifies Metabotropic Glutamate Receptor 1 in Proportion to the Regional Density of Its Gene Transcript in Human Brain

A recent study from our laboratory found that (18)F-FIMX is an excellent PET radioligand for quantifying metabotropic glutamate receptor 1 (mGluR1) in monkey brain. This study evaluated the ability of (18)F-FIMX to quantify mGluR1 in humans. A second goal was to use the relative density of mGluR1 gene transcripts in brain regions to estimate specific uptake and nondisplaceable uptake (VND) in each brain region.

METHODS

After injection of 189 ± 3 MBq of (18)F-FIMX, 12 healthy volunteers underwent a dynamic PET scan over 120 min. For 6 volunteers, images were acquired until 210 min. A metabolite-corrected arterial input function was measured from the radial artery. Four other subjects underwent whole-body scanning to estimate radiation exposure.

RESULTS

(18)F-FIMX uptake into the human brain was high (SUV = 4-6 in the cerebellum), peaked at about 10 min, and washed out rapidly. An unconstrained 2-tissue-compartment model fitted the data well, and distribution volume (VT) (mL⋅cm(-3)) values ranged from 1.5 in the caudate to 11 in the cerebellum. A 120-min scan provided stable VT values in all regions except the cerebellum, for which an acquisition time of at least 170 min was necessary. VT values in brain regions correlated well with mGluR1 transcript density, and the correlation suggested that VND of (18)F-FIMX was quite low (0.5 mL⋅cm(-3)). This measure of VND in humans was similar to that from a receptor blocking study in monkeys, after correcting for differences in plasma protein binding. Similar to other (18)F-labeled ligands, the effective dose was about 23 μSv/MBq.

CONCLUSION

(18)F-FIMX can quantify mGluR1 in the human brain with a 120- to 170-min scan. Correlation of brain uptake with the relative density of mGluR1 transcript allows specific receptor binding of a radioligand to be quantified without injecting pharmacologic doses of a blocking agent.

Synthesis, binding affinity, radiolabeling, and microPET evaluation of 4-(2-substituted-4-substituted)-8-(dialkylamino)-6-methyl-1-substituted-3,4-dihydropyrido[2,3-b]pyrazin-2(1H)-ones as ligands for brain corticotropin-releasing factor type-1 (CRF1) receptors

Compounds 1-14 were synthesized in a search for high-affinity CRF1 receptor ligands that could be radiolabeled with (11)C or (18)F for use as positron emission tomography (PET) radiotracers. Derivatives of 2 were developed which contained amide N-fluoroalkyl substituents. Compounds [(18)F]24 and [(18)F]25 were found to have appropriate lipophilicities of logP7.4=2.2 but microPET imaging with [(18)F]25 demonstrated limited brain uptake.

Neuroimaging assessment of early and late neurobiological sequelae of traumatic brain injury: implications for CTE

Traumatic brain injury (TBI) has been increasingly accepted as a major external risk factor for neurodegenerative morbidity and mortality. Recent evidence indicates that the resultant chronic neurobiological sequelae following head trauma may, at least in part, contribute to a pathologically distinct disease known as Chronic Traumatic Encephalopathy (CTE). The clinical manifestation of CTE is variable, but the symptoms of this progressive disease include impaired memory and cognition, affective disorders (i.e., impulsivity, aggression, depression, suicidality, etc.), and diminished motor control. Notably, mounting evidence suggests that the pathology contributing to CTE may be caused by repetitive exposure to subconcussive hits to the head, even in those with no history of a clinically evident head injury. Given the millions of athletes and military personnel with potential exposure to repetitive subconcussive insults and TBI, CTE represents an important public health issue. However, the incidence rates and pathological mechanisms are still largely unknown, primarily due to the fact that there is no in vivo diagnostic tool. The primary objective of this manuscript is to address this limitation and discuss potential neuroimaging modalities that may be capable of diagnosing CTE in vivo through the detection of tau and other known pathological features. Additionally, we will discuss the challenges of TBI research, outline the known pathology of CTE (with an emphasis on Tau), review current neuroimaging modalities to assess the potential routes for in vivo diagnosis, and discuss the future directions of CTE research.

Tau imaging in the study of ageing, Alzheimer's disease, and other neurodegenerative conditions

In vivo tau imaging allows a deeper understanding of tau deposition in the brain, providing insights into the causes, diagnosis and treatment of primary and secondary tauopathies such as Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, chronic traumatic encephalopathy, and some variants of frontotemporal lobar degeneration. The cross-sectional and longitudinal assessment of the temporal and spatial patterns of tau deposition in the brain will allow a better understanding of the role tau plays in ageing as well as its relationship with cognition, genotype, and neurodegeneration. It is likely that selective tau imaging could be used as a diagnostic and prognostic biomarker of disease progression, as well as a surrogate marker for monitoring of efficacy and patient recruitment for disease-specific therapeutic trials.

PET tracers for serotonin receptors and their applications

Serotonin receptors (5-HTRs) are implicated in the pathophysiology of a variety of neuropsychiatric and neurodegenerative disorders and are also targets for drug therapy. In the CNS, most of these receptors are expressed in high abundance in specific brain regions reflecting their role in brain functions. Quantifying binding to 5-HTRs in vivo may permit assessment of physiologic and pathologic conditions, and monitoring disease progression, evaluating treatment response, and for investigating new treatment modalities. Positron emission tomography (PET) molecular imaging has the sensitivity to quantify binding of 5-HTRs in CNS disorders and to measure drug occupancy as part of a process of new drug development. Although research on PET imaging of 5-HTRs have been performed more than two decades, the successful radiotracers so far developed for human studies are limited to 5-HT₁AR, 5-HT₁BR, 5-HT₂AR, 5-HT₄R and 5-HT₆R. Herein we review the development and application of radioligands for PET imaging of 5-HTRs in living brain.

Methods to Increase the Metabolic Stability of (18)F-Radiotracers

The majority of pharmaceuticals and other organic compounds incorporating radiotracers that are considered foreign to the body undergo metabolic changes in vivo. Metabolic degradation of these drugs is commonly caused by a system of enzymes of low substrate specificity requirement, which is present mainly in the liver, but drug metabolism may also take place in the kidneys or other organs. Thus, radiotracers and all other pharmaceuticals are faced with enormous challenges to maintain their stability in vivo highlighting the importance of their structure. Often in practice, such biologically active molecules exhibit these properties in vitro, but fail during in vivo studies due to obtaining an increased metabolism within minutes. Many pharmacologically and biologically interesting compounds never see application due to their lack of stability. One of the most important issues of radiotracers development based on fluorine-18 is the stability in vitro and in vivo. Sometimes, the metabolism of ¹⁸F-radiotracers goes along with the cleavage of the C-F bond and with the rejection of [¹⁸F]fluoride mostly combined with high background and accumulation in the skeleton. This review deals with the impact of radiodefluorination and with approaches to stabilize the C-F bond to avoid the cleavage between fluorine and carbon.

Identification of a major radiometabolite of [11C]PBB3

INTRODUCTION

[(11)C]PBB3 is a clinically used positron emission tomography (PET) probe for in vivo imaging of tau pathology in the brain. Our previous study showed that [(11)C]PBB3 was rapidly decomposed to a polar radiometabolite in the plasma of mice. For the pharmacokinetic evaluation of [(11)C]PBB3 it is important to elucidate the characteristics of radiometabolites. In this study, we identified the chemical structure of a major radiometabolite of [(11)C]PBB3 and proposed the metabolic pathway of [(11)C]PBB3.

METHODS

Carrier-added [(11)C]PBB3 was injected into a mouse for in vivo metabolite analysis. The chemical structure of a major radiometabolite was identified using LC-MS. Mouse and human liver microsomes and liver S9 samples were incubated with [(11)C]PBB3 in vitro. In silico prediction software was used to assist in the determination of the metabolite and metabolic pathway of [(11)C]PBB3.

RESULTS

In vivo analysis showed that the molecular weight of a major radiometabolite of [(11)C]PBB3, which was called as [(11)C]M2, was m/z 390 [M+H(+)]. In vitro analysis assisted by in silico prediction showed that [(11)C]M2, which was not generated by cytochrome P450 enzymes (CYPs), was generated by sulfated conjugation mediated by a sulfotransferase.

CONCLUSION

The major radiometabolite, [(11)C]M2, was identified as a sulfated conjugate of [(11)C]PBB3. [(11)C]PBB3 was metabolized mainly by a sulfotransferase and subsidiarily by CYPs.

Synthesis, F-18 radiolabeling, and microPET evaluation of 3-(2,4-dichlorophenyl)-N-alkyl-N-fluoroalkyl-2,5-dimethylpyrazolo[1,5-a]pyrimidin-7-amines as ligands of the corticotropin-releasing factor type-1 (CRF1) receptor

A series of 3-(2,4-dichlorophenyl)-N-alkyl-N-fluoroalkyl-2,5-dimethylpyrazolo[1,5-a]pyrimidin-7-amines were synthesized and evaluated as potential positron emission tomography (PET) tracers for the corticotropin-releasing factor type-1 (CRF1) receptor. Compounds 27, 28, 29, and 30 all displayed high binding affinity (⩽1.2 nM) to the CRF1 receptor when assessed by in vitro competition binding assays at 23 °C, whereas a decrease in affinity (⩾10-fold) was observed with compound 26. The logP7.4 values of [(18)F]26-[(18)F]29 were in the range of ∼2.2-2.8 and microPET evaluation of [(18)F]26-[(18)F]29 in an anesthetized male cynomolgus monkey demonstrated brain penetrance, but specific binding was not sufficient enough to differentiate regions of high CRF1 receptor density from regions of low CRF1 receptor density. Radioactivity uptake in the skull, and sphenoid bone and/or sphenoid sinus during studies with [(18)F]28, [(18)F]28-d8, and [(18)F]29 was attributed to a combination of [(18)F]fluoride generated by metabolic defluorination of the radiotracer and binding of intact radiotracer to CRF1 receptors expressed on mast cells in the bone marrow. Uptake of [(18)F]26 and [(18)F]27 in the skull and sphenoid region was rapid but then steadily washed out which suggests that this behavior was the result of binding to CRF1 receptors expressed on mast cells in the bone marrow with no contribution from [(18)F]fluoride.

Exploration of the impact of stereochemistry on the identification of the novel translocator protein PET imaging agent [(18)F]GE-180

INTRODUCTION

The tricyclic indole compound, [(18)F]GE-180 has been previously identified as a promising positron emission tomography (PET) imaging agent of the translocator protein (TSPO) with the potential to aid in the diagnosis, prognosis and therapy monitoring of degenerative neuroinflammatory conditions such as multiple sclerosis. [(18)F]GE-180 was first identified and evaluated as a racemate, but subsequent evaluations of the resolved enantiomers have shown that the S-enantiomer has a higher affinity for TSPO and an improved in vivo biodistribution performance, in terms of higher uptake in specific brain regions and good clearance (as described previously). Here we describe the additional biological evaluations carried out to confirm the improved performance of the S-enantiomer and including experiments which have demonstrated the stability of the chiral centre to chemical and biological factors.

MATERIALS AND METHODS

GE-180 and the corresponding radiolabelling precursor were separated into single enantiomers using semi-preparative chiral supercritical fluid chromatography (SFC). A detailed comparison of the individual enantiomers and the racemate was carried out in a number of biological studies. TSPO binding affinity was assessed using a radioligand binding assay. Incubation with rat hepatic S9 fractions was used to monitor metabolic stability. In vivo biodistribution studies up to 60 min post injection (PI) in naïve rats were carried out to monitor uptake and clearance. Achiral and chiral in vivo metabolite detection methods were developed to assess the presence of metabolite/s in plasma and brain samples, with the chiral method also determining potential racemisation at the chiral centre.

RESULTS

Evaluation of the chiral stability of the two enantiomers to metabolism by rat S9 fractions, showed no racemisation of enantiomers. There were notable differences in the biodistribution between the racemate and the R- and S-enantiomers. All compounds had similar initial brain uptake between 0.99 and 1.01% injected dose (id) at 2 min PI, with S-[(18)F]GE-180 showing significantly greater retention than the R-enantiomer at 10 and 30 min PI (P<0.05). S-[(18)F]GE-180 uptake to the TSPO-expressing olfactory bulbs was 0.45% id (SD ± 0.17) at 30 min PI in comparison to RS-[(18)F]GE-180 or R-[(18)F]GE-180 levels of 0.41% id ± 0.09 and 0.23% id ± 0.02 respectively, at the same timepoint (P > 0.05). The signal-to-noise ratio (ratio olfactory bulb to striata binding) were similar for both RS-[(18)F]GE-180 and S-[(18)F]GE-180 (3.2 and 3.4 respectively). Initial uptake to the lungs (an organ with high TSPO expression) was more than 3-fold greater with S-[(18)F]GE-180 than R-[(18)F]GE-180, and significantly higher at 10 and 30 min PI (P < 0.05). Furthermore lung uptake of S-[(18)F]GE-180 at 2 and 10 min PI was also significant when compared to the racemate (P < 0.05). The majority of the radioactivity in the rat brain following administration of RS-[(18)F]GE-180 or S-[(18)F]GE-180 was due to the presence of the parent compound (91% ± 1.5 and 94% ± 2.0 of total radioactivity at 60 min PI respectively). In contrast at 60 min PI for the plasma samples, the parent compounds accounted for only 28% ± 1.2 and 21% ± 4.6 of total radioactivity for RS-[(18)F]GE-180 and S-[(18)F]GE-180 respectively. Chiral assessment confirmed that the S-enantiomer was chirally stable in vivo, with no stereochemical conversion in brain and plasma samples up to 60 min PI.

CONCLUSIONS

Developing racemic radiotracers, as for racemic therapeutics, is a considerable challenge due to differences of the enantiomers in pharmacokinetics, efficacy and potential toxicity. We have shown that the enantiomers of the promising racemic PET ligand [(18)F]GE-180 do not share identical performance, with S-[(18)F]GE-180 demonstrating higher TSPO affinity, higher brain uptake and better retention to the high TSPO-expressing lungs. Furthermore, S-[(18)F]GE-180 has also been shown to be enantiomerically stable in vivo, with no observed conversation of the eutomer to the distomer. As a single enantiomer, S-[(18)F]GE-180 retains the beneficial characteristics of the racemate and is a promising imaging agent for imaging neuroinflammation in vivo.

Design of a serotonin 4 receptor radiotracer with decreased lipophilicity for single photon emission computed tomography

With the aim to develop a suitable radiotracer for the brain imaging of the serotonin 4 receptor subtype (5-HT4R) using single photon emission computed tomography (SPECT), we synthesized and evaluated a library of di- and triazaphenanthridines with lipophilicity values which were in the range expected to favour brain penetration, and which demonstrated specific binding to the target of interest. Adding additional nitrogen atoms to previously described phenanthridine ligands exhibiting a high unspecific binding, we were able to design a radioiodinated compound [(125)I]14. This compound exhibited a binding affinity value of 0.094 nM toward human 5-HT4R and a high selectivity over other serotonin receptor subtypes (5-HTR). In vivo SPECT imaging studies and competition experiments demonstrated that the decreased lipophilicity (in comparison with our previously reported compounds 4 and 5) allowed a more specific labelling of the 5-HT4R brain-containing regions.

Neuroimaging as a New Diagnostic Modality in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by progressive degeneration of upper and lower motor neurons, with variable involvement of extramotor brain regions. Currently, there are no established objective markers of upper motor neuron and extramotor involvement in ALS. Here, we review the potential diagnostic value of advanced neuroimaging techniques that are increasingly being used to study the brain in ALS. First, we discuss the role of different imaging modalities in our increasing understanding of ALS pathogenesis, and their potential to contribute to objective upper motor neuron biomarkers for the disease. Second, we discuss the challenges to be overcome and the required phases of diagnostic test development to translate imaging technology to clinical care. We also present examples of multidimensional imaging approaches to achieve high levels of diagnostic accuracy. Last, we address the role of neuroimaging in clinical therapeutic trials. Advanced neuroimaging techniques will continue to develop and offer significant opportunities to facilitate the development of new effective treatments for ALS.

In vivo tau imaging: obstacles and progress

The military conflicts of the last decade have highlighted the growing problem of traumatic brain injury in combatants returning from the battlefield. The considerable evidence pointing at the accumulation of tau aggregates and its recognition as a risk factor in neurodegenerative conditions such as Alzheimer's disease have led to a major effort to develop selective tau ligands that would allow research into the physiopathologic underpinnings of traumatic brain injury and chronic traumatic encephalopathy in military personnel and the civilian population. These tracers will allow new insights into tau pathology in the human brain, facilitating research into causes, diagnosis, and treatment of traumatic encephalopathy and major neurodegenerative dementias, such as Alzheimer's disease and some variants of frontotemporal lobar degeneration, in which tau plays a role. The field of selective tau imaging has to overcome several obstacles, some of them associated with the idiosyncrasies of tau aggregation and others related to radiotracer design. A worldwide effort has focused on the development of imaging agents that will allow selective tau imaging in vivo. Recent progress in the development of these tracers is enabling the noninvasive assessment of the extent of tau pathology in the brain, eventually allowing the quantification of changes in tau pathology over time and its relation to cognitive performance, brain volumetrics, and other biomarkers, as well as assessment of efficacy and patient recruitment for antitau therapeutic trials.

[(18)F]FDG is not transported by P-glycoprotein and breast cancer resistance protein at the rodent blood-brain barrier

INTRODUCTION

Transport of 2-[(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG) by the multidrug efflux transporters P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) at the blood-brain barrier (BBB) may confound the interpretation of [(18)F]FDG brain PET data. Aim of this study was to assess the influence of ABCB1 and ABCG2 at the BBB on brain distribution of [(18)F]FDG in vivo by performing [(18)F]FDG PET scans in wild-type and transporter knockout mice and by evaluating changes in [(18)F]FDG brain distribution after transporter inhibition.

METHODS

Dynamic small-animal PET experiments (60min) were performed with [(18)F]FDG in groups of wild-type and transporter knockout mice (Abcb1a/b((-/-)), Abcg2((-/-)) and Abcb1a/b((-/-))Abcg2((-/-))) and in wild-type rats without and with i.v. pretreatment with the known ABCB1 inhibitor tariquidar (15mg/kg, given at 2h before PET). Blood was sampled from animals from the orbital sinus vein at the end of the PET scans and measured in a gamma counter. Brain uptake of [(18)F]FDG was expressed as the brain-to-blood radioactivity concentration ratio in the last PET time frame (Kb,brain).

RESULTS

Kb,brain values of [(18)F]FDG were not significantly different between different mouse types both without and with tariquidar pretreatment. The blood-to-brain transfer rate constant of [(18)F]FDG was significantly lower in tariquidar-treated as compared with vehicle-treated rats (0.350±0.025mL/min/g versus 0.416±0.024mL/min/g, p=0.026, paired t-test) but Kb,brain values were not significantly different between both rat groups.

CONCLUSION

Our results show that [(18)F]FDG is not transported by Abcb1 at the mouse and rat BBB in vivo. In addition we found no evidence for Abcg2 transport of [(18)F]FDG at the mouse BBB.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Our findings imply that functional activity of ABCB1 and ABCG2 at the BBB does not need to be taken into account when interpreting brain [(18)F]FDG PET data.

Tau imaging: early progress and future directions

Use of selective in-vivo tau imaging will enable improved understanding of tau aggregation in the brain, facilitating research into causes, diagnosis, and treatment of major tauopathies such as Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, chronic traumatic encephalopathy, and some variants of frontotemporal lobar degeneration. Neuropathological studies of Alzheimer's disease show a strong association between tau deposits, decreased cognitive function, and neurodegenerative changes. Selective tau imaging will allow the in-vivo exploration of such associations and measure the global and regional changes in tau deposits over time. Such imaging studies will comprise non-invasive assessment of the spatial and temporal pattern of tau deposition over time, providing insight into the role tau plays in ageing and helping to establish the relation between cognition, genotype, neurodegeneration, and other biomarkers. Once validated, selective tau imaging might be useful as a diagnostic, prognostic, and progression biomarker, and a surrogate marker for the monitoring of efficacy and patient recruitment for anti-tau therapeutic trials.

Preclinical Comparison of the Amyloid-β Radioligands [(11)C]Pittsburgh compound B and [(18)F]florbetaben in Aged APPPS1-21 and BRI1-42 Mouse Models of Cerebral Amyloidosis

PURPOSE

The aim of this study was to compare [(11)C]Pittsburgh compound B ([(11)C]PiB) and [(18)F]florbetaben ([(18)F]FBB) for preclinical investigations of amyloid-β pathology.

PROCEDURES

We investigated two aged animal models of cerebral amyloidosis with contrasting levels of amyloid-β relating to "high" (APPPS1-21 n = 6, wild type (WT) n = 7) and "low" (BRI1-42 n = 6, WT n = 6) target states, respectively.

RESULTS

APPPS1-21 mice (high target state) demonstrated extensive fibrillar amyloid-β deposition that translated to significantly increased retention of [(11)C]PiB and [(18)F]FBB in comparison to their wild type. The retention pattern of [(11)C]PiB and [(18)F]FBB in this cohort displayed a significant correlation. However, the relative difference in tracer uptake between diseased and healthy mice was substantially higher for [(11)C]PiB than for [(18)F]FBB. Although immunohistochemistry confirmed the high plaque load in APPPS1-21 mice, correlation between tracer uptake and ex vivo quantification of amyloid-β was poor for both tracers. BRI1-42 mice (low target state) did not demonstrate increased tracer uptake.

CONCLUSIONS

In cases of high fibrillar amyloid-β burden, both tracers detected significant differences between diseased and healthy mice, with [(11)C]PiB showing a larger dynamic range.