Two activated stages of microglia and PET imaging of peripheral benzodiazepine receptors with [<Superscript>11</Superscript>C]PK11195 in rats

Objective  

The transition of microglia from the normal resting state to the activated state is associated with an increased expression of peripheral benzodiazepine receptors (PBR). The extent of PBR expression is dependent on the level of microglial activation. A PBR ligand, [¹¹C]PK11195, has been used for imaging of the activation of microglia in vivo. We evaluated whether [¹¹C]PK11195 PET can indicate differences of microglial activation between no treatment and lipopolysaccharide (LPS) treatment in a rat artificial injury model of brain inflammation.     

Quantification of (<Emphasis Type="Italic">R</Emphasis>)-[<Superscript>11</Superscript>C]PK11195 binding in rheumatoid arthritis

Purpose  Rheumatoid arthritis (RA) involves migration of macrophages into inflamed areas. (R)-[¹¹C]PK11195 binds to peripheral benzodiazepine receptors, expressed on macrophages, and may be used to quantify inflammation using positron emission tomography (PET). This study evaluated methods for the quantification of (R)-[¹¹C]PK11195 binding in the knee joints of RA patients. Methods  Data from six patients with RA were analysed. Dynamic PET scans were acquired in 3-D mode following (R)-[¹¹C]PK11195 injection. During scanning arterial radioactivity concentrations were measured to determine the plasma (R)-[¹¹C]PK11195 concentrations. Data were analysed using irreversible and reversible one-tissue and two-tissue compartment models and input functions with various types of metabolite correction. Model preferences according to the Akaike information criterion (AIC) and correlations between measures were evaluated. Correlations between distribution volume (V_d) and standardized uptake values (SUV) were evaluated. Results  AIC indicated optimal performance for a one-tissue reversible compartment model including blood volume. High correlations were observed between V_d obtained using different input functions (R ²=0.80–1.00) and between V_d obtained with one- and two-tissue reversible compartment models (R ²=0.75–0.94). A high correlation was observed between optimal V_d and SUV after injection (R ²=0.73). Conclusion  (R)-[¹¹C]PK11195 kinetics in the knee were best described by a reversible single-tissue compartment model including blood volume. Applying metabolite corrections did not increase sensitivity. Due to the high correlation with V_d, SUV is a practical alternative for clinical use. Financial support: There were no sources of financial support other than the VU University Medical Centre. The authors had full control of all primary data and agree to allow EJNM to review their data if requested.     

PET scanning of macrophages in patients with scleroderma fibrosing alveolitis

RationaleAssessment of disease activity in fibrosing alveolitis due to systemic sclerosis (FASSc) is difficult without using invasive investigation. A repeatable noninvasive method of assessing disease at a cellular level such as with positron emission tomography (PET) could be of great value in evaluating high-resolution changes in the pathological process.ObjectivesTo investigate whether the level of inflammatory cell traffic and lung density in FASSc, imaged in vivo by PET, is different to controls and whether they are associated with changes in pulmonary function indices.MethodsWe used PET to measure lung density and tissue uptake of ¹¹C-[R]-PK11195, a ligand that binds to receptors found in abundance in macrophages. Fifteen patients with FASSc were compared to seven controls.ResultsA trend of reduced uptake of ¹¹C-[R]-PK11195 was observed in FASSc patients (P=.09) and correlated inversely with lung density (r=?.62; P<.05), which was significantly elevated in FASSc [0.35±0.02 vs. 0.23±0.02 g/cc (mean±S.E.M.); P<.005].ConclusionThese results demonstrate that inflammatory cell traffic and lung density can be imaged in vivo in FASSc using PET, and that this approach might be of potential value in understanding, in situ, components of pathogenesis that may have value for prognosis.     

Image-derived input functions for PET brain studies

Purpose  To assess the robustness of a previously introduced method to obtain accurate image-derived input functions (IDIF) for three other tracers. Methods  Dynamic PET and online blood data of five repeat [¹¹C]PIB (Pittsburgh Compound-B) ([¹¹C]PIB), six repeat (R)-[¹¹C]verapamil, and ten single (R)-[¹¹C]PK11195 studies were used. IDIFs were extracted from partial volume corrected scans using the four hottest pixels per plane method. Results obtained with IDIFs were compared with those using standard online measured arterial input functions (BSIF). IDIFs were used both with and without calibration based on manual blood samples. Results  For (R)-[¹¹C]verapamil, accurate IDIFs were obtained using noncalibrated IDIFs (slope 0.96±0.17; R ² 0.92±0.07). However, calibration was necessary to obtain IDIFs comparable to the BSIF for both [¹¹C]PIB (slope 1.04±0.05; R ² 1.00±0.01) and (R)-[¹¹C]PK11195 (slope 0.96±0.05; R ² 0.99±0.01). The need for calibration may be explained by the sticking property of both tracers, indicating that BSIF may be affected by sticking and therefore may be unreliable. Conclusion  The present study shows that a previously proposed method to extract IDIFs is suitable for analysing [¹¹C]PIB, (R)-[¹¹C]verapamil and (R)-[¹¹C]PK11195 studies, thereby obviating the need for online arterial sampling.     

Whole-body distribution and metabolism of [N-methyl-11C](R)-1-(2-chlorophenyl)-N-(1-methylpropyl)-3-isoquinolinecarboxamide in humans; an imaging agent for in vivo assessment of peripheral benzodiazepine receptor activity with positron emission tomography

Purpose   ¹¹C-PK11195 is a radiopharmaceutical for in vivo assessment of peripheral benzodiazepine receptor (PBR) activity using PET. We sought to clarify the metabolic fate of ¹¹C-PK11195 in a test–retest setting using radio-HPLC in comparison with radio-TLC, and the whole-body distribution in humans. Materials and methods  In order to evaluate the reproducibility of radio-HPLC metabolite analyses, ten patients with Alzheimer’s disease (AD) underwent two successive ¹¹C-PK11195 examinations on separate days. For comparison of different analytical methods, plasma samples from seven patients were also analysed by radio-TLC. In addition, we evaluated the whole-body distribution of ¹¹C-PK11195 and its uptake in the brain. Results  The level of unmetabolized ¹¹C-PK11195 decreased slowly from 96.3 ± 1.6% (mean±SD) at 5 min to 62.7 ± 8.3% at 40 min after injection. Large individual variation was observed in the amount of plasma ¹¹C-PK11195 radiometabolites. The whole-body distribution of ¹¹C-PK11195 showed the highest radioactivity levels in urinary bladder, adrenal gland, liver, salivary glands, heart, kidneys, and vertebral column. In addition, the hip bone and breast bone were clearly visualized by PET. In patients with AD, ¹¹C-PK11195 uptake in the brain was the highest in the basal ganglia and thalamus, followed by the cortical grey matter regions and the cerebellum. Low ¹¹C-PK11195 uptake was observed in the white matter. Conclusion  Our results indicate that ¹¹C-PK11195 is eliminated both through the renal and hepatobiliary systems. Careful analysis of plasma metabolites is required to determine the accurate arterial input function for quantitative PET measurement.     

In vivo changes in microglial activation and amyloid deposits in brain regions with hypometabolism in Alzheimer��s disease

Purpose

Amyloid ?? protein (A??) is known as a pathological substance in Alzheimer??s disease (AD) and is assumed to coexist with a degree of activated microglia in the brain. However, it remains unclear whether these two events occur in parallel with characteristic hypometabolism in AD in vivo. The purpose of the present study was to clarify the in vivo relationship between A?? accumulation and neuroinflammation in those specific brain regions in early AD.

Methods

Eleven nootropic drug-na?ve AD patients underwent a series of positron emission tomography (PET) measurements with [¹¹C](R)PK11195, [¹¹C]PIB and [¹⁸F]FDG and a battery of cognitive tests within the same day. The binding potentials (BPs) of [¹¹C](R)PK11195 were directly compared with those of [¹¹C]PIB in the brain regions with reduced glucose metabolism.

Results

BPs of [¹¹C](R)PK11195 and [¹¹C]PIB were significantly higher in the parietotemporal regions of AD patients than in ten healthy controls. In AD patients, there was a negative correlation between dementia score and [¹¹C](R)PK11195 BPs, but not [¹¹C]PIB, in the limbic, precuneus and prefrontal regions. Direct comparisons showed a significant negative correlation between [¹¹C](R)PK11195 and [¹¹C]PIB BPs in the posterior cingulate cortex (PCC) (p?<?0.05, corrected) that manifested the most severe reduction in [¹⁸F]FDG uptake.

Conclusion

A lack of coupling between microglial activation and amyloid deposits may indicate that A?? accumulation shown by [¹¹C]PIB is not always the primary cause of microglial activation, but rather the negative correlation present in the PCC suggests that microglia can show higher activation during the production of A?? in early AD.     

In vivo imaging of neuroinflammation: a comparative study between [18F]PBR111, [11C]CLINME and [11C]PK11195 in an acute rodent model

Purpose

The key role of neuroinflammation in acute and chronic neurological disorders has stimulated the search for specific radiotracers targeting the peripheral benzodiazepine receptor (PBR)/18 kDa translocator protein (TSPO), a hallmark of neuroinflammation. Here we evaluate the new radiotracer for positron emission tomography (PET) [¹⁸F]PBR111 in a rodent model of acute inflammation and compare it with [¹¹C]CLINME, an ¹¹C-labelled tracer of the same chemical family, and with the isoquinolinic carboxamide [¹¹C]PK11195.

Methods

We studied radiometabolites by HPLC, in vitro binding by autoradiography and in vivo brain kinetics as well as in vivo specificity of binding using PET imaging.

Results

We show that this radiotracer has a high in vitro specificity for PBR/TSPO versus central benzodiazepine receptors, as reflected by the drastic reduction of its binding to target tissue by addition of PK11195 or PBR111, while addition of flumazenil does not affect binding. Only intact [¹⁸F]PBR111 is detected in brain up to 60 min after i.v. injection, and PET imaging shows an increased uptake in the lesion as compared to the contralateral side as early as 6 min after injection. Administration of an excess of PK11195 and PBR111, 20 min after [¹⁸F]PBR111 administration, induces a rapid and complete displacement of [¹⁸F]PBR111 binding from the lesion. Modelling of the PET data using the simplified reference tissue model showed increased binding potential (BP) in comparison to [¹¹C]PK11195.

Conclusion

[¹⁸F]PBR111 is a metabolically stable tracer with a high specific in vitro and in vivo binding to TSPO. In addition, considering the longer half-life of ¹⁸F over ¹¹C, these results support [¹⁸F]PBR111 as a promising PET tracer of the PBR/TSPO for neuroinflammation imaging.     

Uptake of inflammatory cell marker [<Superscript>11</Superscript>C]PK11195 into mouse atherosclerotic plaques

Purpose  The ligand [¹¹C]PK11195 binds with high affinity and selectivity to peripheral benzodiazepine receptor, expressed in high amounts in macrophages. In humans, [¹¹C]PK11195 has been used successfully for the in vivo imaging of inflammatory processes of brain tissue. The purpose of this study was to explore the feasibility of [¹¹C]PK11195 in imaging inflammation in the atherosclerotic plaques. Methods  The presence of PK11195 binding sites in the atherosclerotic plaques was verified by examining the in vitro binding of [³H]PK11195 onto mouse aortic sections. Uptake of intravenously administered [¹¹C]PK11195 was studied ex vivo in excised tissue samples and aortic sections of a LDLR/ApoB48 atherosclerotic mice. Accumulation of the tracer was compared between the atherosclerotic plaques and non-atherosclerotic arterial sites by autoradiography and histological analyses. Results  The [³H]PK11195 was found to bind to both the atherosclerotic plaques and the healthy wall. The autoradiography analysis revealed that the uptake of [¹¹C]PK11195 to inflamed regions in plaques was more prominent (p = 0.011) than to non-inflamed plaque regions, but overall it was not higher than the uptake to the healthy vessel wall. Also, the accumulation of ¹¹C radioactivity into the aorta of the atherosclerotic mice was not increased compared to the healthy control mice. Conclusions  Our results indicate that the uptake of [¹¹C]PK11195 is higher in inflamed atherosclerotic plaques containing a large number of inflammatory cells than in the non-inflamed plaques. However, the tracer uptake to other structures of the artery wall was also prominent and may limit the use of [¹¹C]PK11195 in clinical imaging of atherosclerotic plaques.     

[11C]-(R)PK11195 tracer kinetics in the brain of glioma patients and a comparison of two referencing approaches

Purpose

Translocator protein (TSPO) is a biomarker of neuroinflammation that can be imaged by PET using [¹¹C]-(R)PK11195. We sought to characterize the [¹¹C]-(R)PK11195 kinetics in gliomas of different histotypes and grades, and to compare two reference tissue input functions (supervised cluster analysis versus cerebellar grey matter) for the estimation of [¹¹C]-(R)PK11195 binding in gliomas and surrounding brain structures.

Methods

Twenty-three glioma patients and ten age-matched controls underwent structural MRI and dynamic [¹¹C]-(R)PK11195 PET scans. Tissue time–activity curves (TACs) were extracted from tumour regions as well as grey matter (GM) and white matter (WM) of the brains. Parametric maps of binding potential (BP_ND) were generated with the simplified reference tissue model using the two input functions, and were compared with each other. TSPO expression was assessed in tumour tissue sections by immunohistochemistry.

Results

Three types of regional kinetics were observed in individual tumour TACs: GM-like kinetics (n?=?6, clearance of the tracer similar to that in cerebellar GM), WM-like kinetics (n?=?8, clearance of the tracer similar to that in cerebral WM) and a form of mixed kinetics (n?=?9, intermediate rate of clearance). Such kinetic patterns differed between low-grade astrocytomas (WM-like kinetics) and oligodendrogliomas (GM-like and mixed kinetics), but were independent of tumour grade. There was good agreement between parametric maps of BP_ND derived from the two input functions in all controls and 10 of 23 glioma patients. In 13 of the 23 patients, BP_ND values derived from the supervised cluster input were systematically smaller than those using the cerebellar input. Immunohistochemistry confirmed that TSPO expression increased with tumour grade.

Conclusion

The three types of [¹¹C]-(R)PK11195 kinetics in gliomas are determined in part by tracer delivery, and indicated that kinetic analysis is a valuable tool in the study of gliomas with the potential for in vivo discrimination between low-grade astrocytomas and oligodendrogliomas. Supervised cluster and cerebellar input functions produced consistent BP_ND estimates in approximately half of the gliomas investigated, but had a systematic difference in the remainder. The cerebellar input is preferred based on theoretical and practical considerations.     

Carbidopa pretreatment improves image interpretation and visualisation of carcinoid tumours with 11C-5-hydroxytryptophan positron emission tomography

Purpose Positron emission tomography (PET) with ¹¹C-5-hydroxytryptophan (5-HTP) as tracer is a promising imaging instrument in the management of patients with neuroendocrine tumours (NETs). However, high radioactivity concentrations in the urinary collecting system sometimes produce image reconstruction artefacts that can make detection of small NETs difficult. As a means to decrease urinary excretion of radioactivity and thereby improve image quality, we examined the effect of pretreatment with carbidopa (CD), a peripheral inhibitor of aromatic amino acid decarboxylase (AADC), which converts 5-HTP to serotonin (5-hydroxytryptamine, 5-HT).Methods Six patients with midgut carcinoid metastases were examined with ¹¹C-5-HTP PET before and 1 h after oral administration of 100 or 200 mg of CD.Results There was a fourfold significant reduction of tracer uptake in the urinary collecting system after CD administration (p=0.0277, n=6), with a mean standard uptake value (SUV) of 155±195 before CD and 39±14 after CD. In tumour lesions there was a significant increase in SUV after CD administration (p<0. 0001, n=18), with a mean SUV of 11±3 before CD and 14±3 after CD. There was no difference between the doses (100 and 200 mg) of CD in this respect. In all patients, image interpretation and tumour detection were markedly improved after CD administration.Conclusion We conclude that CD premedication improves ¹¹C-5-HTP PET image quality and facilitates detection of NET lesions. Because of the similarity of metabolic pathways, this method could probably be applied to improve PET imaging using other tracers like ¹⁸F-DOPA and ¹¹C-DOPA.     

Evaluation of [11C]-DAA1106 for imaging and quantification of neuroinflammation in a rat model of herpes encephalitis

IntroductionMany neurological and psychiatric disorders are associated with neuroinflammation. Positron emission tomography (PET) with [¹¹C]-PK11195 can be used to study neuroinflammation in these disorders. However, [¹¹C]-PK11195 may not be sensitive enough to visualize mild neuroinflammation. As a potentially more sensitive PET tracer for neuroinflammation, [¹¹C]-N-(2,5-dimethoxybenzyl)-N-(4-fluoro-2-phenoxyphenyl)-acetamide (DAA1106) was evaluated in a rat model of herpes encephalitis.MethodsMale Wistar rats were intranasally inoculated with HSV-1 (HSE) or phosphate-buffered saline (control). At Day 6 or Day 7 after inoculation, small-animal [¹¹C]-DAA1106 PET scans were acquired, followed by ex vivo biodistribution. Arterial blood sampling was performed for quantification of uptake.ResultsIn HSE rats, a significantly higher ex vivo, but not in vivo, uptake of [¹¹C]-DAA1106 was found in almost all examined brain areas (24–71%, P<.05), when compared to control rats. Pretreatment with unlabeled PK11195 effectively reduced [¹¹C]-DAA1106 uptake in HSE rats (54–84%; P<.001). The plasma and brain time–activity curves showed rapid uptake of [¹¹C]-DAA1106 into tissue. The data showed a good fit to the Logan analysis but could not be fitted to a two-tissue compartment model.Conclusions[¹¹C]-DAA1106 showed a high and specific ex vivo uptake in the encephalitic rat brain. However, neuroinflammation could not be demonstrated in vivo by [¹¹C]-DAA1106 PET. Quantification of the uptake of [¹¹C]-DAA1106 using plasma sampling is not optimal, due to rapid tissue uptake, slow tissue clearance and low plasma activity.     

A Single Intraperitoneal Injection of Endotoxin Changes Glial Cells in Rats as Revealed by Positron Emission Tomography Using [<Superscript>11</Superscript>C]PK11195

Purpose

Intracranial administration of lipopolysaccharide (LPS) is known to elicit a rapid innate immune response, activate glial cells in the brain, and induce depression-like behavior. However, no study has focused on the changes in glial cells induced by intraperitoneal injection of LPS in vivo.

Methods

Ten adult male Fischer F344 rats underwent [¹¹C]PK11195 PET before and 2 days after intraperitoneal injection of LPS to evaluate the changes in glial cells. The difference in standardized uptake values (SUV) of [¹¹C]PK11195 between before and after injection was determined.

Results

There was a cluster of brain regions that showed significant reductions in SUV. This cluster included the bilateral striata and bilateral frontal regions, especially the somatosensory areas.

Conclusions

Changes in activity of glial cells induced by the intraperitoneal injection of LPS were detected in vivo by [¹¹C]PK11195 PET. Intraperitoneal injection of LPS is known to induce depression, and further studies with [¹¹C]PK11195 PET would clarify the relationships between neuroinflammation and depression.

     

Validation of cardiac 123I-MIBG scintigraphy in patients with Parkinson’s disease who were diagnosed with dopamine PET

Purpose  

The aim of this study was to evaluate the diagnostic potential of cardiac ¹²³I-labelled metaiodobenzylguanidine (¹²³I-MIBG) scintigraphy in idiopathic Parkinson’s disease (PD). The diagnosis was confirmed by positron emission tomography (PET) imaging with ¹¹C-labelled 2β-carbomethoxy-3β-(4-fluorophenyl)-tropane (¹¹C-CFT) and ¹¹C-raclopride (together designated as dopamine PET).     

In vivo imaging of system xc- as a novel approach to monitor multiple sclerosis

Purpose

Glutamate excitotoxicity contributes to oligodendroglial and axonal damage in multiple sclerosis pathology. Extracellular glutamate concentration in the brain is controlled by cystine/glutamate antiporter (system xc-), a membrane antiporter that imports cystine and releases glutamate. Despite this, the system xc^? activity and its connection to the inflammatory reaction in multiple sclerosis (MS) is largely unknown.

Methods

Longitudinal in vivo magnetic resonance (MRI) and positron emission tomography (PET) imaging studies with 2-[¹⁸F]Fluoro-2-deoxy-D-glucose ([¹⁸F]FDG), [¹¹C]-(R)-(1-(2-chlorophenyl)-N-methyl-N-1(1-methylpropyl)-3-isoquinolinecarboxamide ([¹¹C]PK11195) and (4S)-4-(3-¹⁸F-fluoropropyl)-L-glutamate ([¹⁸F]FSPG) were carried out during the course of experimental autoimmune encephalomyelitis (EAE) induction in rats.

Results

[¹⁸F]FSPG showed a significant increase of system xc^? function in the lumbar section of the spinal cord at 14 days post immunization (dpi) that stands in agreement with the neurological symptoms and ventricle edema formation at this time point. Likewise, [¹⁸F]FDG did not show significant changes in glucose metabolism throughout central nervous system and [¹¹C]PK11195 evidenced a significant increase of microglial/macrophage activation in spinal cord and cerebellum 2 weeks after EAE induction. Therefore, [¹⁸F]FSPG showed a major capacity to discriminate regions of the central nervous system affected by the MS in comparison to [¹⁸F]FDG and [¹¹C]PK11195. Additionally, clodronate-treated rats showed a depletion in microglial population and [¹⁸F]FSPG PET signal in spinal cord confirming a link between neuroinflammatory reaction and cystine/glutamate antiporter activity in EAE rats.

Conclusions

Altogether, these results suggest that in vivo PET imaging of system xc^? could become a valuable tool for the diagnosis and treatment evaluation of MS.

     

Microglial activation in perinatal rabbit brain induced by intrauterine inflammation: detection with 11C-(R)-PK11195 and small-animal PET.

Intrauterine infection can lead to a fetal inflammatory response syndrome that has been implicated as one of the causes of perinatal brain injury leading to periventricular leukomalacia (PVL) and cerebral palsy. The presence of activated microglial cells has been noted in autopsy specimens of patients with PVL and in models of neonatal hypoxia and ischemia. Activated microglial cells can cause oligodendrocyte damage and white matter injury by release of inflammatory cytokines and production of excitotoxic metabolites. We hypothesized that exposure to endotoxin in utero leads to microglial activation in the fetal brain that can be monitored in vivo by (11)C-(R)-PK11195 (1-[2-chlorophenyl]-N-methyl-N-[1-methylpropyl]-3-isoquinoline carboxamide)--a positron-emitting ligand that binds peripheral benzodiazepine receptor sites in activated microglia--using small-animal PET. METHODS: Pregnant New Zealand White rabbits underwent laparotomy and were injected with 20 and 30 microg/kg of Escherichia coli lipopolysaccharide along the length of the uterus on day 28 of gestation. The pups were born spontaneously at term (31 d) and were scanned using small-animal PET after intravenous administration of (11)C-(R)-PK11195 and by MRI on postnatal day 1. The standard uptake values (SUVs) of the tracer were calculated for the whole brain at 10-min intervals for 60 min after tracer injection. The pups were euthanized after the scan, and brains were fixed, sectioned, and stained for microglial cells using biotinylated tomato lectin. RESULTS: There was increased brain retention of (11)C-(R)-PK11195--as determined by a significant difference in the slope of the SUV over time--in the endotoxin-treated pups when compared with that of age-matched controls. Immunohistochemical staining showed dose-dependent changes in activated microglia (increased number and morphologic changes) in the periventricular region and hippocampus of the brain of newborn rabbit pups exposed to endotoxin in utero. CONCLUSION: Intrauterine inflammation leads to activation of microglial cells that may be responsible for the development of brain injury and white matter damage in the perinatal period. PET with the tracer (11)C-(R)-PK11195 can be used as a noninvasive, sensitive tool for determining the presence and progress of neuroinflammation due to perinatal insults in newborns.     

Reproducibility of automated simplified voxel-based analysis of PET amyloid ligand [<Superscript>11</Superscript>C]PIB uptake using 30-min scanning data

Purpose  

Positron emission tomography (PET) with ¹¹C-labelled Pittsburgh compound B ([¹¹C]PIB) enables the quantitation of β-amyloid accumulation in the brain of patients with Alzheimer’s disease (AD). Voxel-based image analysis techniques conducted in a standard brain space provide an objective, rapid and fully automated method to analyze [¹¹C]PIB PET data. The purpose of this study was to evaluate both region- and voxel-level reproducibility of automated and simplified [¹¹C]PIB quantitation when using only 30 min of imaging data.     

Biodistribution and radiation dosimetry of [11C]choline: a comparison between rat and human data

Purpose  

Methyl-¹¹C-choline ([¹¹C]choline) is a radiopharmaceutical used for oncological PET studies. We investigated the biodistribution and biokinetics of [¹¹C]choline and provide estimates of radiation doses in humans.     

In vivo imaging of the 18-kDa translocator protein (TSPO) with [18F]FEDAA1106 and PET does not show increased binding in Alzheimer’s disease patients

Purpose

Imaging the 18-kDa translocator protein (TSPO) is considered a potential tool for in vivo evaluation of microglial activation and neuroinflammation in the early stages of Alzheimer’s disease (AD). ((R)-1-(2-chlorophenyl)-N-[¹¹C]-methyl-N-(1-methylpropyl)-3-isoquinoline caboxamide ([¹¹C]-(R)-PK11195) has been widely used for PET imaging of TSPO and, despite its low specific-to-nondisplaceable binding ratio, increased TSPO binding has been shown in AD patients. The high-affinity radioligand N-(5-fluoro-2-phenoxyphenyl)-N-(2-[¹⁸F]fluoroethyl-5-methoxybenzyl)acetamide ([¹⁸F]FEDAA1106) has been developed as a potential in vivo imaging tool for better quantification of TSPO binding. The aim of this study was to quantify in vivo binding of [¹⁸F]FEDAA1106 to TSPO in control subjects and AD patients.

Methods

Seven controls (five men, two women, age 68±3 years, MMSE score 29±1) and nine AD patients (six men, three women, age 69±4 years, MMSE score 25±3) were studied with [¹⁸F]FEDAA1106. PET measurements were performed on an ECAT EXACT HR system (Siemens Medical Solutions) in two 60-min dynamic PET sessions with a 30-min interval between sessions. Arterial blood radioactivity was measured using an automated blood sampling system for the first 5 min and using manually drawn samples thereafter. Quantification was performed using both kinetic analysis based on a two-tissue compartment model and Logan graphical analysis. Outcome measures were total distribution volume (V _T) and binding potential (BP _ND=k ₃/k ₄). An estimate of nondisplaceable distribution volume was obtained with the Logan graphical analysis using the first 15 min of PET measurements (V _{ND 1-15 min}). Binding potential (BP _ND) was also calculated as: V _T/V _{ND 1-15 min} ? 1.

Results

No statistically significant differences in V _T, k ₃/k ₄ or BP _ND were observed between controls and AD patients.

Conclusion

This study suggests that TSPO imaging with [¹⁸F]FEDAA1106 does not enable the detection of microglial activation in AD.     

Preclinical and the first clinical studies on [11C]CHIBA-1001 for mapping α7 nicotinic receptors by positron emission tomography

Objective  4-[¹¹C]methylphenyl 2,5-diazabicyclo[3.2.2]nonane-2-carboxylate ([¹¹C]CHIBA-1001), a 4-methyl-substituted derivative of the selective α7 nicotinic acetylcholine receptor (α7 nAChR) partial agonist 4-bromophenyl 1,4 diazabicyclo[3.2.2]nonane-4-carboxylate (SSR180711), is a potential radioligand for mapping α7 nAChRs in the brain by positron emission tomography (PET). In this study, we performed preclinical and first clinical PET studies using [¹¹C]CHIBA-1001 for imaging α7 nAChRs in the human brain. Methods  [¹¹C]CHIBA-1001 was synthesized by methylation of the tributylstannyl precursor with [¹¹C]CH₃I in a palladium-promoted Stille cross-coupling reaction. The radiation absorbed-dose of [¹¹C]CHIBA-1001 in humans was calculated from distribution data in mice. The acute toxicity of CHIBA-1001 at a dose of 3.20 mg/kg body weight, which is more than 41,000-fold the clinical equivalent dose of [¹¹C]CHIBA-1001, was evaluated. The mutagenicity of CHIBA-1001 was studied by a reverse mutation test in Salmonella typhimurium (Ames test). Metabolite analysis in the mouse brain was carried out by high-performance liquid chromatography. The first clinical PET imaging of α7 nAChRs with [¹¹C]CHIBA-1001 in a normal volunteer was also performed. Results  A suitable preparation method for [¹¹C]CHIBA-1001 injection was established. The radiation absorbed-dose by [¹¹C]CHIBA-1001 in humans was low enough for clinical use, and no acute toxicity or mutagenicity of CHIBA-1001 was found. Most radioactivity in the mouse brain was detected as an unchanged form, although peripherally [¹¹C]CHIBA-1001 was degraded. We successfully performed brain imaging by PET with [¹¹C]CHIBA-1001 in a normal volunteer. A 90-min dynamic scan showed a rapid accumulation and gradual washout of radioactivity in the brain. The highest distribution volume of [¹¹C]CHIBA-1001 was found in the thalamus; however, regional differences in brain radioactivity were small. Peripherally, [¹¹C]CHIBA-1001 was stable in humans: >80% of the radioactivity in plasma was detected as the unchanged form for 60 min. Conclusions  These results demonstrate that [¹¹C]CHIBA-1001 is a suitable radioligand to use in clinical trials for imaging α7 nAChRs in the human brain, providing acceptable dosimetry and pharmacological safety at the dose required for adequate PET imaging.     

Evaluation of CLINDE as potent translocator protein (18 kDa) SPECT radiotracer reflecting the degree of neuroinflammation in a rat model of microglial activation

Background  The translocator protein (TSPO; 18 kDa), the new name of the peripheral-type benzodiazepine receptor, is localised in mitochondria of glial cells and expressed in very low concentrations in normal brain. Their expression rises after microglial activation following brain injury. Accordingly, TSPO are potential targets to evaluate neuroinflammatory changes in a variety of CNS disorders. Purpose  To date, only a few effective tools are available to explore TSPO by SPECT. We characterised here 6-chloro-2-(4′iodophenyl)-3-(N,N-diethyl)-imidazo[1,2-a]pyridine-3-acetamide or CLINDE in a rat model with different stages of excitotoxic lesion. Methods  Excitotoxicity was induced in male Wistar rats by unilateral intrastriatal injection of different amounts of quinolinic acid (75, 150 or 300 nmol). Six days later, two groups of rats (n = 5–6/group) were i.v. injected with [¹²⁵I]-CLINDE (0.4 MBq); one group being pre-injected with PK11195 (5 mg/kg). Brains were removed 30 min after tracer injection and the radioactivity of cerebral areas measured. Complementary ex vivo autoradiography, in vitro autoradiography ([³H]-PK11195) and immunohistochemical studies (OX-42) were performed on brain sections. Results  In the control group, [¹²⁵I]-CLINDE binding was significantly higher (p < 0.001) in lesioned than that in intact side. This binding disappeared in rats pre-treated with PK11195 (p < 0.001), showing specific binding of CLINDE to TSPO. Ex vivo and in vitro autoradiographic studies and immunohistochemistry were consistent with this, revealing a spatial correspondence between radioactivity signal and activated microglia. Regression analysis yielded a positive relation between the ligand binding and the degree of neuroinflammation. Conclusion  These results demonstrate that CLINDE is suitable for TSPO in vivo SPECT imaging to explore their involvement in neurodegenerative disorders associated with microglial activation. An erratum to this article can be found at