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.     

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.

     

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.     

Imaging of peripheral-type benzodiazepine receptor in tumor: in vitro binding and in vivo biodistribution of N-benzyl-N-[C]methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide

IntroductionThe aim of this study was to evaluate N-benzyl-N-[¹¹C]methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ([¹¹C]DAC) as a novel peripheral-type benzodiazepine receptor (PBR) ligand for tumor imaging.Methods[¹¹C]DAC was synthesized by the reaction of a desmethyl precursor with [¹¹C]CH₃I. In vitro uptake of [¹¹C]DAC was examined in PBR-expressing C6 glioma and intact murine fibrosarcoma (NFSa) cells. In vivo distribution of [¹¹C]DAC was determined using NFSa-bearing mice and small-animal positron emission tomography (PET).Results[¹¹C]DAC showed specific binding to PBR in C6 glioma cells, a standard cell line with high PBR expression. Specific binding of [¹¹C]DAC was also confirmed in NFSa cells, a target tumor cell line in this study. Results of PET experiments using NFSa-bearing mice, showed that [¹¹C]DAC was taken up specifically into the tumor, and pretreatment with PK11195 abolished the uptake.Conclusions[¹¹C]DAC was taken up into PBR-expressing NFSa. [¹¹C]DAC is a promising PET ligand that can be used for imaging PBR in tumor-bearing mice.     

Radiosynthesis and evaluation of [11C]YM-202074 as a PET ligand for imaging the metabotropic glutamate receptor type 1

IntroductionDeveloping positron emission tomography (PET) ligands for imaging metabotropic glutamate receptor type 1 (mGluR1) is important for studying its role in the central nervous system. N-cyclohexyl-6-{[N-(2-methoxyethyl)-N-methylamino]methyl}-N-methylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-202074) exhibited high binding affinity for mGluR1 (K_i=4.8 nM), and selectivity over other mGluRs in vitro. The purpose of this study was to label YM-202074 with carbon-11 and to evaluate in vitro and in vivo characteristics of [¹¹C]YM-202074 as a PET ligand for mGluR1 in rodents.Methods[¹¹C]YM-202074 was synthesized by N-[¹¹C]methylation of its desmethyl precursor with [¹¹C]methyl iodide. The in vitro and in vivo brain regional distributions were determined in rats using autoradiography and PET, respectively.Results[¹¹C]YM-202074 (262–630 MBq, n=5) was obtained with radiochemical purity of >98% and specific activity of 27–52 GBq/μmol at the end of synthesis, starting from [¹¹C]CO₂ of 19.3–21.5 GBq. In vitro autoradiographic results showed that the high specific binding of [¹¹C]YM-202074 for mGluR1 was presented in the cerebellum, thalamus and hippocampus, which are known as mGluR1-rich regions. In ex vivo autoradiography and PET studies, the radioligand was specifically distributed in the cerebellum, although the uptake was low. Furthermore, the regional distribution was fairly uniform in the whole brain by pretreatment with JNJ16259685 (a mGluR1 antagonist). However, radiometabolite(s) was detected in the brain.ConclusionsFrom these results, especially considering the low brain uptake and the influx of radiometabolite(s) into brain, [¹¹C]YM-202074 may not be a useful PET ligand for in vivo imaging of mGluR1 in the brain.     

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 in the rodent brain with [11C]SSR180575, a novel indoleacetamide radioligand of the translocator protein (18?kDa)

Purpose

Neuroinflammation is involved in neurological disorders through the activation of microglial cells. Imaging of neuroinflammation with radioligands for the translocator protein (18 kDa) (TSPO) could prove to be an attractive biomarker for disease diagnosis and therapeutic evaluation. The indoleacetamide-derived 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide, SSR180575, is a selective high-affinity TSPO ligand in human and rodents with neuroprotective effects.

Methods

Here we report the radiolabelling of SSR180575 with ¹¹C and in vitro and in vivo imaging in an acute model of neuroinflammation in rats.

Results

The image contrast and the binding of [¹¹C]SSR180575 are higher than that obtained with the isoquinoline-based TSPO radioligand, [¹¹C]PK11195. Competition studies demonstrate that [¹¹C]SSR180575 has high specific binding for the TSPO.

Conclusion

[¹¹C]SSR180575 is the first PET radioligand for the TSPO based on an indoleacetamide scaffold designed for imaging neuroinflammation in animal models and in the clinic.     

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.     

Nuclear imaging of neuroinflammation: a comprehensive review of [11C]PK11195 challengers

Neurodegenerative, inflammatory and neoplastic brain disorders involve neuroinflammatory reactions, and a biomarker of neuroinflammation would be useful for diagnostic, drug development and therapy control of these frequent diseases. In vivo imaging can document the expression of the peripheral benzodiazepine receptor (PBR)/translocator protein 18 kDa (TSPO) that is linked to microglial activation and considered a hallmark of neuroinflammation. The prototype positron emission tomography tracer for PBR, [¹¹C]PK11195, has shown limitations that until now have slowed the clinical applications of PBR imaging. In recent years, dozens of new PET and SPECT radioligands for the PBR have been radiolabelled, and several have been evaluated in imaging protocols. Here we review the new PBR ligands proposed as challengers of [¹¹C]PK11195, critically analyze preclinical imaging studies and discuss their potential as neuroinflammation imaging agents.     

Radiosynthesis and ex vivo evaluation of (R)-(−)-2-chloro-N-[1-11C-propyl]n-propylnorapomorphine

IntroductionSeveral dopamine D₂ agonist radioligands have been used with positron emission tomography (PET), including [¹¹C-]-(?)-MNPA, [¹¹C-]-(?)-NPA and [¹¹C]-(+)-PHNO. These radioligands are considered particularly powerful for detection of endogenous dopamine release, but they either provide PET brain images with limited contrast or have affinity for both D₂ and D₃ receptors. We here present the carbon-11 radiolabeling and ex vivo evaluation of 2-Cl-(?)-NPA, a novel PET-tracer candidate with high in vitro D₂/D₃ selectivity.Methods2-Cl-[¹¹C]-(?)-NPA and [¹¹C]-(?)-NPA were synthesized by a two step N-acylation-reduction process using [¹¹C]-propionyl chloride. Awake rats were injected with either tracer, via the tail vein. The rats were decapitated at various times, the brains were removed and quickly dissected, and plasma metabolites were measured. Radioligand specificity, and P-glycoprotein involvement in brain uptake, was also assessed.Results2-Cl-[¹¹C]-(?)-NPA and [¹¹C]-(?)-NPA were produced in high specific activity and purity. 2-Cl-[¹¹C]-(?)-NPA accumulated slower in the striatum than [¹¹C]-(?)-NPA, reaching maximum concentrations after 30 min. The maximal striatal uptake of 2-Cl-[¹¹C]-(?)-NPA (standard uptake value 0.72±0.24) was approximately half that of [¹¹C]-(?)-NPA (standard uptake value 1.37±0.18). Nonspecific uptake was similar for the two compounds. 2-Cl-[¹¹C]-(?)-NPA was metabolized quickly, leaving only 17% of the parent compound in the plasma after 30 min. The specific binding of 2-Cl-[¹¹C]-(?)-NPA was completely blocked and inhibition of P-glycoprotein did not alter the brain uptake.ConclusionEx vivo experiments showed, despite a favorable D₂/D₃ selectivity, that 2-Cl-[¹¹C]-(?)-NPA is inferior to [¹¹C]-(?)-NPA as a PET tracer in rat, because of slower brain uptake and lower specific to nonspecific binding ratio.     

Radiation dosimetry and biodistribution of the translocator protein radiotracer [11C]DAA1106 determined with PET/CT in healthy human volunteers

IntroductionWhen microglia become activated (an integral part of neuroinflammation), cellular morphology changes and expression of translocator protein (TSPO) 18 kDa is increased. Over the past several years, [¹¹C]DAA1106 has emerged as a reliable radiotracer for labeling TSPO with high affinity during positron emission tomography (PET) scanning. While [¹¹C]DAA1106 PET scanning has been used in several research studies, a radiation dosimetry study of this radiotracer in humans has not yet been published.MethodsTwelve healthy participants underwent full body dynamic [¹¹C]DAA1106 PET scanning, with 8 sequential whole body scans (approximately 12 bed positions each), following a single injection. Regions of interest were drawn manually, and time activity curves (TACs) were obtained for 15 organs. OLINDA/EXM 1.1 was used to compute radiation absorbed doses to the target organs, as well as effective dose (ED) and effective dose equivalent (EDE).ResultsThe ED and EDE were 4.06 ± 0.58 μSv/MBq and 5.89 ± 0.83 μSv/MBq, respectively. The highest absorbed doses were to the heart wall, kidney, liver, pancreas, and spleen. TACs revealed that peak dose rates are during the first scan (at 6 min) for all organs other than the urinary bladder wall, which had its peak dose rate during the fourth scan (at 30 min).ConclusionsThe recently developed radiotracer [¹¹C]DAA1106 has its EDE and target-organ absorbed dose such that, for a single administration, its radiation dosimetry is well within the U.S. FDA guidelines for basic research studies in adults. This dose level implies that the dosimetry for multiple [¹¹C]DAA1106 scans within a given year also falls within FDA guidelines, and this favorable property makes this radiotracer suitable for examining microglial activation repeatedly over time, which may in the future be useful for longitudinal tracking of disease progression and monitoring of therapy response in conditions marked by neuroinflammation (e.g., head trauma and multiple sclerosis).     

Human biodistribution and radiation dosimetry of 11C-(R)-PK11195, the prototypic PET ligand to image inflammation

Purpose  

The positron emission tomography (PET) radiotracer ¹¹C-(R)-PK11195 allows the in vivo imaging in humans of the translocator protein 18 kDa (TSPO), previously called peripheral benzodiazepine receptor (PBR), a marker of inflammation. Despite its widespread use, the radiation burden associated with ¹¹C-(R)-PK11195 in humans is not known. To examine this, we performed dynamic whole-body imaging with PET and ¹¹C-(R)-PK11195 in healthy humans.     

[11 C]Rhodamine-123: Synthesis and biodistribution in rodents

IntroductionRhodamine-123 is a known substrate for the efflux transporter, P-glycoprotein (P-gp). We wished to assess whether rhodamine-123 might serve as a useful substrate for developing probes for imaging efflux transporters in vivo with positron emission tomography (PET). For this purpose, we aimed to label rhodamine-123 with carbon-11 (t_1/2 = 20.4 min) and to study its biodistribution in rodents.Methods[¹¹ C]Rhodamine-123 was prepared by treating rhodamine-110 (desmethyl-rhodamine-123) with [¹¹ C]methyl iodide. The biodistribution of this radiotracer was studied with PET in wild-type mice and rats, in efflux transporter knockout mice, in wild-type rats pretreated with DCPQ (an inhibitor of P-gp) or with cimetidine (an inhibitor of organic cation transporters; OCT), and in P-gp knockout mice pretreated with cimetidine. Unchanged radiotracer in forebrain, plasma and peripheral tissues was also measured ex vivo at 30 min after radiotracer administration to wild-type and efflux transporter knockout rodents.Results[¹¹ C]Rhodamine-123 was obtained in 4.4% decay-corrected radiochemical yield from cyclotron-produced [¹¹ C]carbon dioxide. After intravenous administration of [¹¹ C]rhodamine-123 to wild-type rodents, PET and ex vivo measurements showed radioactivity uptake was very low in brain, but relatively high in some other organs such as heart, and especially liver and kidney. Inhibition of P-gp increased uptake in brain, heart, kidney and liver, but only by up to twofold. Secretion of radioactivity from kidney was markedly reduced by OCT knockout or pretreatment with cimetidine.Conclusions[¹¹ C]Rhodamine-123 was unpromising as a PET probe for P-gp function and appears to be a strong substrate of OCT in kidney. Cimetidine appears effective for blocking OCT in kidney in vivo.     

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.     

The translocator protein ligand [18F]DPA-714 images glioma and activated microglia in vivo

Purpose

In recent years there has been an increase in the development of radioligands targeting the 18-kDa translocator protein (TSPO). TSPO expression is well documented in activated microglia and serves as a biomarker for imaging neuroinflammation. In addition, TSPO has also been reported to be overexpressed in a number of cancer cell lines and human tumours including glioma. Here we investigated the use of [¹⁸F]DPA-714, a new TSPO positron emission tomography (PET) radioligand to image glioma in vivo.

Methods

We studied the uptake of [¹⁸F]DPA-714 in three different rat strains implanted with 9L rat glioma cells: Fischer (F), Wistar (W) and Sprague Dawley (SD) rats. Dynamic [¹⁸F]DPA-714 PET imaging, kinetic modelling of PET data and in vivo displacement studies using unlabelled DPA-714 and PK11195 were performed. Validation of TSPO expression in 9L glioma cell lines and intracranial 9L gliomas were investigated using Western blotting and immunohistochemistry of brain tissue sections.

Results

All rats showed significant [¹⁸F]DPA-714 PET accumulation at the site of 9L tumour implantation compared to the contralateral brain hemisphere with a difference in uptake among the three strains (F?>?W?>?SD). The radiotracer showed high specificity for TSPO as demonstrated by the significant reduction of [¹⁸F]DPA-714 binding in the tumour after administration of unlabelled DPA-714 or PK11195. TSPO expression was confirmed by Western blotting in 9L cells in vitro and by immunohistochemistry ex vivo.

Conclusion

The TSPO radioligand [¹⁸F]DPA-714 can be used for PET imaging of intracranial 9L glioma in different rat strains. This preclinical study demonstrates the feasibility of employing [¹⁸F]DPA-714 as an alternative radiotracer to image human glioma.     

Tumor uptake of 68Ga-DOTA-Tyr3-octreotate: animal PET studies of tumor flow and acute somatostatin receptor modulation in the CA20948 rat model

IntroductionFactors determining the in vivo uptake of radiolabeled somatostatin analogs by neuroendocrine tumors are poorly known. The aim is to evaluate in vivo tumor perfusion and regulation of somatostatin receptors (sstr) following acute exposure to octreotide, in an animal model of neuroendocrine tumor.MethodsH₂¹⁵O flow studies were performed in 8 CA20948 tumor-bearing rats and another 36 rats underwent three [⁶⁸Ga]-DOTA-Tyr³-octreotate imaging sessions at 24-h intervals. After baseline (Day 0) imaging, scanning was repeated on Day 1 after octreotide injection (175 μg/kg), with a variable delay: no injection (controls, n=7), coinjection (n=6), and octreotide injection 20 min (n=7), 2 h (n=8) and 4 h (n=8) before imaging. Repeat images without octreotide was performed at Day 2 followed by sacrifice and tumor counting.ResultsH₂¹⁵O studies failed to measure quantitative tumor perfusion in this model. On Day 1, ratio of tumor uptake to Day 0 was 1.2±0.3 in controls; 0.6±0.2 in the coinjection group; 0.9±0.2, 1.1±0.1 and 1.2±0.2 in the other groups, respectively. Uptake in the coinjection group showed a statistically significant reduction of tumor uptake (P<.0001). All groups showed increased uptake on Day 2, without statistical differences between groups. In vivo tumor counts showed good correlation with ex vivo countings (R²=0.946).ConclusionAcute exposure to unlabeled octreotide in this neuroendocrine tumor model results in a rapid recycling or resynthesis of sstr. Positron emission tomography (PET) allowed to reliably assess quantitative uptake of [⁶⁸Ga]-DOTA-Tyr³-octreotate over time in the same animal, but failed in this model to measure tumor perfusion.     

PET imaging of neuroinflammation in a rat traumatic brain injury model with radiolabeled TSPO ligand DPA-714

Purpose

The inflammatory response in injured brain parenchyma after traumatic brain injury (TBI) is crucial in the pathological process. In order to follow microglia activation and neuroinflammation after TBI, we performed PET imaging in a rat model of TBI using ¹⁸F-labeled DPA-714, a ligand of the 18-kDa translocator protein (TSPO).

Methods

TBI was induced in male SD rats by a controlled cortical impact. The success of the TBI model was confirmed by MRI. [¹⁸F]DPA-714 was synthesized using a slightly modified TRACERLab FX-FN module and an automated procedure. In vivo PET imaging was performed at different time points after surgery using an Inveon small-animal PET scanner. The specificity of [¹⁸F]DPA-714 was confirmed by a displacement study with an unlabeled competitive TSPO ligand, PK11195. Ex vivo autoradiography as well as immunofluorescence staining was carried out to confirm the in vivo PET results.

Results

Both in vivo T₂-weighted MR images and ex vivo TTC staining results revealed successful establishment of the TBI model. Compared with the sham-treated group, [¹⁸F]DPA-714 uptake was significantly higher in the injured brain area on PET images. Increased lesion-to-normal ratios of [¹⁸F]DPA-714 were observed in the brain of TBI rats on day 2 after surgery. Ratios peaked around day 6 (2.65?±?0.36) and then decreased gradually to nearly normal levels on day 28. The displacement study using PK11195 confirmed the specific binding of [¹⁸F]DPA-714 to TSPO. The results of ex vivo autoradiography were consistent with in vivo PET results. Immunofluorescence staining showed the time course of TSPO expression after TBI and the temporal and the spatial distribution of microglia in the damaged brain area.

Conclusion

TSPO-targeted PET using [¹⁸F]DPA-714 as the imaging probe can be used to dynamically monitor the inflammatory response after TBI in a noninvasive manner. This method will not only facilitate a better understanding of the inflammatory process after TBI, but also provide a useful in vivo monitoring strategy for antiinflammation therapy of TBI.     

Radiosynthesis and ex vivo evaluation of [11C-carbonyl]carbamate- and urea-based monoacylglycerol lipase inhibitors

IntroductionMonoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) are the two primary enzymes that regulate the tone of endocannabinoid signaling. Although new PET radiotracers have been discovered for imaging FAAH in vivo, no such radiotracer exists for imaging MAGL. Here we report the radiosynthesis of five candidate MAGL radiotracers and their ex vivo evaluations in mice and rats.MethodsCandidate carbamate and urea MAGL inhibitors were radiolabeled at the carbonyl position by [¹¹C]CO₂ fixation. Radiotracers were administered (tail-vein injection) to rodents and brain uptake of radioactivity measured at early and late time points ex vivo. Specificity of uptake was explored by pretreatment with unlabeled inhibitors (2 mg/kg, ip) 30 min prior to radiotracer administration.ResultsAll five candidate MAGL radiotracers were prepared in high specific activity (> 65 GBq/μmol) and radiochemical purity (> 98%). Moderate brain uptake (0.2–0.8 SUV) was observed for each candidate while pretreatment did not reduce uptake for four of the five tested. For two candidates ([¹¹C]12 and [¹¹C]14), high retention of radioactivity was observed in the blood (ca. 10 and 4 SUV at 40 min) which was blocked by pretreatment with unlabeled inhibitors. The most promising candidate, [¹¹C]18, demonstrated moderate brain uptake (ca. 0.8 SUV) which showed circa 50% blockade by pretreatment with unlabeled 18.ConclusionOne putative and four reported potent and selective MAGL inhibitors have been radiolabeled via [¹¹C]CO₂ fixation as radiotracers for this enzyme. Despite the promising in vitro pharmacological profile, none of the five candidate radiotracers exhibited in vivo behavior suitable for PET neuroimaging.     

Systemic catechol-O-methyl transferase inhibition enables the D1 agonist radiotracer R-[11C]SKF 82957

IntroductionR-[¹¹C]-SKF 82957 is a high-affinity and potent dopamine D₁ receptor agonist radioligand, which gives rise to a brain-penetrant lipophilic metabolite. In this study, we demonstrate that systemic administration of catechol-O-methyl transferase (COMT) inhibitors blocks this metabolic pathway, facilitating the use of R-[¹¹C]-SKF 82957 to image the high-affinity state of the dopamine D₁ receptor with PET.MethodsR-[¹¹C]SKF 82957 was administered to untreated and COMT inhibitor-treated conscious rats, and the radioactive metabolites present in the brain and plasma were quantified by HPLC. Under optimal conditions, cerebral uptake and dopamine D₁ binding of R-[¹¹C]SKF 82957 were measured ex vivo. In addition, pharmacological challenges with the receptor antagonist SCH 23390, amphetamine, the dopamine reuptake inhibitor RTI-32 and the dopamine hydroxylase inhibitor α-methyl-p-tyrosine were performed to study the specificity and sensitivity of R-[¹¹C]-SKF 82957 dopamine D₁ binding in COMT-inhibited animals.ResultsTreatment with the COMT inhibitor tolcapone was associated with a dose-dependent (EC₉₀ 5.3±4.3 mg/kg) reduction in the lipophilic metabolite. Tolcapone treatment (20 mg/kg) also resulted in a significant increase in the striatum/cerebellum ratio of R-[¹¹C]SKF 82957, from 15 (controls) to 24. Treatment with the dopamine D₁ antagonist SCH 23390 reduced the striatal binding to the levels of the cerebellum, demonstrating a high specificity and selectivity of R-[¹¹C]SKF 82957 binding.ConclusionsPre-treatment with the COMT inhibitor tolcapone inhibits formation of an interfering metabolite of R-[¹¹C]SKF 82957. Under such conditions, R-[¹¹C]SKF 82957 demonstrates high potential as the first agonist radiotracer for imaging the dopamine D₁ receptor by PET.     

Synthesis and evaluation of new imaging agent for central nicotinic acetylcholine receptor α7 subtype

IntroductionThe nicotinic acetylcholine receptor (nAChR) α7 subtype (α₇ nAChR) is one of the major nAChR subtypes in the brain. We synthesized C-11 labeled α₇ nAChR ligands, (R)-2-[¹¹C]methylamino-benzoic acid 1-aza-bicyclo[2.2.2]oct-3-yl ester ([¹¹C](R)-MeQAA) and its isomer (S)-[¹¹C]MeQAA, for in vivo investigation with positron emission tomography (PET). Then, the potential of (R)- and (S)-[¹¹C]MeQAA for in vivo imaging of α₇ nAChR in the brain was evaluated in mice and monkeys.MethodsThe binding affinity for α₇ nAChR was measured using rat brain. Biodistribution and in vivo receptor blocking studies were undertaken in mice. Dynamic PET scans were performed in conscious monkeys.ResultsThe affinity for α₇ nAChR was 41 and 182 nM for (R)- and (S)-MeQAA, respectively. The initial uptake in the mouse brain was high ([¹¹C](R)-MeQAA: 7.68 and [¹¹C](S)-MeQAA: 6.65 %dose/g at 5 min). The clearance of [¹¹C](R)-MeQAA was slow in the hippocampus (α₇ nAChR-rich region) but was rapid in the cerebellum (α₇ nAChR-poor region). On the other hand, the clearance was fast for [¹¹C](S)-MeQAA in all regions. The brain uptake of [¹¹C](R)-MeQAA was decreased by methyllycaconitine (α₇ nAChR antagonist) treatment. In monkeys, α₇ nAChRs were highly distributed in the thalamus and cortex but poorly distributed in the cerebellum. The high accumulation was observed in the cortex and thalamus for [¹¹C](R)-MeQAA, while the uptake was rather homogeneous for [¹¹C](S)-MeQAA.Conclusions[¹¹C](R)-MeQAA was successfully synthesized and showed high uptake to the brain. However, since the in vivo selectivity for α₇ nAChR was not enough, further PET kinetic analysis or structure optimization is needed for specific visualization of brain α₇ nAChRs in vivo.