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.     

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.     

11C-PK11195的自动化合成及其生物学分布

目的 研究N-[11C]甲基-N-(1-甲基丙基)-1-(2.氯苯基)异喹啉-3-氨甲酰(11C-PK11195)在国内现有合成模块上的自动化合成程序及其在小鼠体内的生物学分布.方法 以1-(2-氯苯基)-N-(1-甲基丙基)-异喹啉-3-氨甲酰去甲基前体与国产模块生产的11C-CH3I在TracerLabFXF-N自动化合成模块中进行甲基化反应,制备11C-PK11195,测定11C-PK11195的纯度和稳定性,观察11C-PK11195在小鼠体内的生物学分布[以每克组织百分注射剂量率(%ID/g)表示]和异常毒性,并进行健康家猫PET显像.结果 从11C-CO2生产到11C-PK11195合成结束总的合成时间约35 min,甲基化合成11C-PK11195的放化产率为(47±3.6)%,其放化纯和化学纯度均>98%,比活度为30～65 GBq/μmol,"C-PK11195注射液室温放置1 h内放化纯>95%.生物学分布实验表明,11C-PK11195在小鼠体内的清除较快,1 min时为(21.44±3.08)%ID/g,60 ndn下降到(1.35±0.54)%ID/g,肾为其主要的排泄器官.注射后1～5 min内,鼠脑放射性水平较高,随后脑内放射性快速下降;心、肺和肾组织中的放射性较高.猫PET显像示肝和肠道摄取最高,其次为肾、肺、大脑、心肌、胃、脾和膀胱,脑组织中放射性分布均匀.结论 该方法可制备出满足临床应用的11C-PK11195,其合成程序也适合于在国内其他模块中应用.11C-PK11195可望用于国内临床PET显像研究.在注射显像剂后30～40 min进行PET显像,可获得较佳PET图像.     

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.     

Peripheral benzodiazepine receptors in the brain of cirrhosis patients with manifest hepatic encephalopathy

Purpose It has been suggested that ammonia-induced enhancement of peripheral benzodiazepine receptors (PBRs) in the brain is involved in the development of hepatic encephalopathy (HE). This hypothesis is based on animal experiments and studies of post-mortem human brains using radiolabelled PK11195, a specific ligand for PBR, but to our knowledge has not been tested in living patients. The aim of the present study was to test this hypothesis by measuring the number of cerebral PBRs in specific brain regions in cirrhotic patients with an acute episode of clinically manifest HE and healthy subjects using dynamic ¹¹C-PK11195 brain PET. Methods Eight cirrhotic patients with an acute episode of clinically manifest HE (mean arterial ammonia 81 μmol/l) and five healthy subjects (22 μmol/l) underwent dynamic ¹¹C-PK11195 and ¹⁵O-H₂O PET, co-registered with MR images. Brain regions (putamen, cerebellum, cortex and thalamus) were delineated on co-registered ¹⁵O-H₂ ¹⁵O and MR images and copied to the dynamic ¹⁵O-H₂O and ¹¹C-PK11195 images. Regional cerebral blood flow (CBF) (¹⁵O-H₂O scan) and the volume of distribution of PK11195 (¹¹C-PK11195 scan) were calculated by kinetic analysis. Results There were regional differences in the CBF, with lowest values in the cortex and highest values in the putamen in both groups of subjects (p<0.05), but no significant differences between the groups. There were no significant differences in the volume of distribution of PK11195 (V _d) between regions or between the two groups of subjects. Mean values of V _d ranged from 1.0 to 1.1 in both groups of subjects. Conclusion The results do not confirm the hypothesis of an increased number of PBRs in patients with HE.     

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.     

11C-AC-5216: a novel PET ligand for peripheral benzodiazepine receptors in the primate brain.

Developing a PET ligand for imaging of the peripheral benzodiazepine receptor (PBR; Translocator Protein [18 kDa] TSPO) is of great importance for studying its role in glial cells in the injured brain and in neurodegenerative disorders, such as Alzheimer's disease. The aim of this study was to synthesize and evaluate N-benzyl-N-ethyl-2-(7-(11)C-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ((11)C-AC-5216) as a PET ligand for imaging PBR in the primate brain. METHODS: AC-5216 and its desmethyl precursor (compound 1) were synthesized starting from commercially available compounds. The radiosynthesis of (11)C-AC-5216 was performed through the reaction of compound 1 with (11)C-CH(3)I in the presence of NaH. The in vivo brain regional distribution was determined in mice (dissection) and a monkey (PET). RESULTS: (11)C-AC-5216 (800-1,230 MBq; n = 25) was obtained with a radiochemical purity of 98% and a specific activity of 85-130 GBq/mumol at the end of synthesis. After injection of (11)C-AC-5216 into mice, a high accumulation of radioactivity was found in the lungs, heart, adrenal glands, and other PBR-rich organs. In the mouse brain, high radioactivity was observed in the olfactory bulb and cerebellum. Radioactivity in these regions was inhibited by nonradioactive AC-5216 or PK11195 but was not decreased by central benzodiazepine receptor-selective flumazenil and Ro15-4513. A PET study of the monkey brain determined that (11)C-AC-5216 had a relatively high uptake in the occipital cortex, a rich PBR-dense area in the primate brain. Pretreatment with nonradioactive AC-5216 and PK11195 reduced the radioactivity of (11)C-AC-5216 in the occipital cortex significantly, suggesting its high specific binding with PBR in the brain. Metabolite analysis demonstrated that (11)C-AC-5216 was stable in vivo in the mouse brain, although it was metabolized in the plasma of mice and the monkey. CONCLUSION: (11)C-AC-5216 is a promising PET ligand for imaging PBR in rodent and primate brains.     

PET study of carbon-11-PK 11195 binding to peripheral type benzodiazepine sites in glioblastoma: a case report.

The utility of the peripheral type benzodiazepine site ligand 11C-PK 11195, for imaging human glioma in conjunction with Positron Emission Tomography, relies on a high specific binding of the tracer to tumoral peripheral type benzodiazepines sites. In a patient with glioblastoma, we found that 11C-PK 11195 binding was two-fold higher in the tumor than in normal gray matter and that 30% of tumoral binding could be displaced by a large excess of unlabeled drug. These findings suggest that tumoral retention of the ligand is due, in part, to specific binding.     

Synthesis and evaluation of11C-PK 11195 forin vivo study of peripheral-type benzodiazepine receptors using position emission tomography

The biodistribution of 3H-PK 11195, an antagonist of the peripheral-type benzodiazepine receptors, was studied in mice. High accumulations of radioactivity in the heart, lung, spleen, kidney and adrenal were observed after intravenous injection of tracer amounts of 3H-PK 11195 into the mice. The radioactivity in the heart, lung, spleen, kidney and adrenal was significantly decreased by the coadministration of carrier PK 11195, which indicated that PK 11195 specifically binds to the receptors. No radioactive metabolites were observed in the heart, lung and brain 20 min after intravenous administration of 3H-PK 11195. The accumulation of 3H-PK 11195 in the lung was not affected by pretreatment with either alpha-methyl benzylamine or imipramine, suggesting that 3H-PK 11195 specifically binds to the receptors. The ratios of radioactivity of the kidney, adrenal and spleen to blood increased as a function of time, whereas that of the lung and heart rapidly reached to a steady state. 11C-PK 11195 was synthesized by the N-methylation of desmethyl precursor yielding more than 100 mCi with high specific activity (more than 1.4 Ci/mumol). The labeling and purification procedure was completed within 23 min after the end of bombardment (EOB). The 11C-PK 11195 solution for injection seems to have a high potential for the in vivo study of the peripheral-type benzodiazepine receptors in the living human by means of positron emission tomography (PET).     

Extrastriatal spreading of microglial activation in Parkinson’s disease: a positron emission tomography study

Background

The neuroinflammatory glial response contributes to the degenerative process in Parkinson’s disease (PD). However, the pattern of microglial progression remains unclear.

Methods

We evaluated microglial activation in early stage PD patients by quantifying changes in neuroinflammation using PET with [¹¹C]DPA713, a selective PET tracer for microglial activation. Eleven PD patients (Hoehn and Yahr stages 1–2) without dementia underwent the [¹¹C]DPA713 PET scan two times with 1 year apart. The binding potential (BP_ND) was estimated with the simplified reference tissue model. Voxelwise and regions of interest analyses were used to compare the regional BP_ND among groups.

Results

Significant increase in [¹¹C]DPA713 BP_ND was found extrastriatally in the occipital, temporal and parietal cortex in PD patients, and the degree of BP_ND became much higher over the brain regions predominantly in the temporal and occipital cortex 1 year later.

Conclusion

The current results indicated that an extrastriatal spreading of microglial activation reflects one of PD pathophysiology occurring at an early stage.

     

Comparing amyloid-β deposition,neuroinflammation, glucose metabolism,and mitochondrial complex I activity in brain: a PET study in aged monkeys

Purpose

The aim of the present study was to compare amyloid-β (Aβ) deposition, translocator protein (TSPO) activity, regional cerebral metabolic rate of glucose (rCMRglc), and mitochondrial complex I (MC-I) activity in the brain of aged monkeys.

Methods

PET scans with ¹¹C-PIB (Aβ), ¹⁸F-BCPP-EF (MC-I), ¹¹C-DPA-713 (TSPO), and ¹⁸F-FDG (rCMRglc) were performed in aged monkeys (Macaca mulatta) in the conscious state and under isoflurane anaesthesia. ¹¹C-PIB binding to Aβ and ¹¹C-DPA-713 binding to TSPO were evaluated in terms of standard uptake values (SUV). The total volume of distribution (V _T) of ¹⁸F-BCPP-EF and rCMRglc with ¹⁸F-FDG were calculated using arterial blood sampling.

Results

Isoflurane did not affect MC-I activity measured in terms of ¹⁸F-BCPP-EF uptake in living brain. There was a significant negative correlation between ¹⁸F-BCPP-EF binding (V _T) and ¹¹C-PIB uptake (SUVR), and there was a significant positive correlation between ¹¹C-DPA-713 uptake (SUV) and ¹¹C-PIB uptake. In contrast, there was no significant correlation between rCMRglc ratio and ¹¹C-PIB uptake.

Conclusion

¹⁸F-BCPP-EF could be a potential PET probe for quantitative imaging of impaired MC-I activity that is correlated with Aβ deposition in the living brain.     

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.     

The apparent positive cooperativity of in vivo [3H]PK-11195 binding in mouse fibrosarcoma

To evaluate the binding properties of peripheral benzodiazepine receptor (PBR) in mouse fibrosarcoma, [(3)H]PK-11195 binding, in vitro and in vivo, was investigated using either tissue dissection or autoradiographic method. The binding characteristics in fibrosarcoma were compared with those in the kidney. The results of an in vitro saturation study revealed that the maximal numbers of PBR binding sites (B(max)) in fibrosarcoma and in the kidney were almost the same (kidney: 5.2 pmol/mg protein; fibrosarcoma: 5.0 pmol/mg protein). On the other hand, the binding affinity (K(d)) in fibrosarcoma was lower than that in the kidney (kidney: 0.45 nM; fibrosarcoma: 1.34 nM). It is noteworthy that the in vivo binding of [(3)H]PK-11195 in fibrosarcoma increased with increasing doses of [(3)H]PK-11195 (in the dose range of 0.03-1 mg/kg), whereas that in the kidney decreased with competitive inhibition. The apparent positive cooperativity of [(3)H]PK-11195 binding in fibrosarcoma was only observed under in vivo conditions and might be possibly related to the incoordination of PBR subunits.     

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.     

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.     

Peripheral-type benzodiazepine receptors in bronchoalveolar lavage cells of patients with interstitial lung disease

INTRODUCTION: PK11195 is a ligand with high affinity for peripheral benzodiazepine receptors (PBRs), which are present in large numbers in macrophages. PBRs play a role in antioxidant pathways and apoptosis, key factors in control of lung health. Intrapulmonary PBRs, assessed in vivo by positron emission tomography (PET), are decreased in interstitial lung disease (ILD) despite increased macrophage numbers. We wished to ascertain whether the observed decrease in in vivo expression of PBRs in the PET scans could be accounted for by a reduction in PBRs per cell by saturation-binding assays of R-PK11195 in cells obtained by bronchoalveolar lavage (BAL). METHODS: We performed receptor saturation-binding assays with [(3)H]-R-PK11195 on a mixed population of cells recovered by BAL to quantify the number of R-PK11195 binding sites per macrophage in 10 subjects with ILD and 10 normal subjects. RESULTS: Receptor affinity [dissociation constant (Kd)] was similar in ILD patients and controls. However, R-PK11195 binding sites per cell [(maximal binding sites available (B(max))] were decreased in macrophages obtained by BAL from subjects with ILD compared to normal (P<.0005). Microautoradiography confirmed localization of R-PK11195 to macrophages in a mixed inflammatory cell population obtained by BAL. CONCLUSION: These results demonstrate that in vitro PBR expression per cell on macrophages obtained by BAL is reduced in patients with ILD indicating a potentially functionally different macrophage phenotype. As PBRs are involved in the orchestration of lung inflammatory responses, this finding offers further insight into the role of macrophages in the pathogenesis of ILDs and offers a potential avenue for pharmacological strategy.     

In vivo imaging of microglial activation using a peripheral benzodiazepine receptor ligand: [11C]PK-11195 and animal PET following ethanol injury in rat striatum

OBJECTIVE: To investigate whether [(11)C]PK-11195, a specific peripheral benzodiazepine receptors (PBRs) ligand for positron emission tomography (PET), can show activated microglia in a rat brain injury model. METHODS: On day 1, ethanol was injected into the rat's right striatum (ST) using a stereotaxic operative procedure. On day 3, head magnetic resonance imaging (MRI) scans for surgically treated rats were performed to evaluate ethanol injury morphologically. On day 4, dynamic PET scans (17 injured rats and 7 non-injured controls) were performed for 60 min with an animal PET scanner under chloral hydrate anesthesia following a bolus injection of [(11)C]PK-11195 through tail vein. Because PBRs are present throughout the brain, there is no suitable receptor-free reference region. The reference tissue model may not be applicable because of low target to background ratio for low affinity of [(11)C]PK-11195 to PBRs. We evaluated the PBRs binding with regions of interest (ROIs)-based approach to estimate total distribution volume (V). We used an integral from 0 min to 60 min (V (60)) as an estimate of V. On the coronal PET image, ROIs were placed on bilateral ST. Differences in right/left ST V (60) ratios between lesioned and unlesioned control rats were compared using unpaired t tests. Immunohistochemical staining was performed for confirming the presence of activated microglia following decapitation on the PET experiment day. RESULTS: The right/left ST V (60) ratios in lesioned rats (1.07 +/- 0.08) were significantly higher than those in unlesioned control rats (1.00 +/- 0.06, P < 0.05). On immunohistochemical staining, activated microglia were exclusively observed in the injured right ST but not in the noninjured left ST of the injury rats and the bilateral ST of the non-injured control rats. CONCLUSIONS: These results suggest that [(11)C]PK-11195 PET imaging would be a useful tool for evaluating microglial activation in a rat brain injury model.     

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.

     

Radiation dosimetry and biodistribution in monkey and man of 11C-PBR28: a PET radioligand to image inflammation.

(11)C-PBR28 ([methyl-(11)C]N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine) is a recently developed radioligand to image peripheral benzodiazepine receptors (PBRs) in brain. The aim of this study was to estimate the human radiation doses of (11)C-PBR28 based on biodistribution data in monkeys and humans. In addition, we scanned 1 human subject who fortuitously behaved as if he lacked the PBR binding protein. METHODS: Whole-body PBR images were acquired after intravenous bolus administration of (11)C-PBR28 in 7 healthy humans (651 +/- 111 MBq) and 2 rhesus monkeys (370 +/- 59.9 MBq). One monkey was scanned after receptor blockade with PK 11195 (10.7 mg/kg intravenously). RESULTS: For typical subjects (subjects 1-6), the 3 organs with highest exposure were those with the high PBR densities (kidneys, spleen, and lungs), and the effective dose was 6.6 microSv/MBq. The unusual subject (subject 7) had 60%-90% less uptake in these 3 organs, resulting in 28% lower effective dose. The activity in the baseline monkey scans was greater than that in humans for organs with high PBR densities. For this reason, the human effective dose was overestimated by 60% with monkey biodistribution data. The monkey with receptor blockade had an overall distribution qualitatively similar to that of the unusual human subject (subject 7), with decreased exposure to lungs, kidney, and spleen. CONCLUSION: The effective dose of (11)C-PBR28 was modest and was similar to that of several other (11)C-radioligands. Lack of receptor binding in the unusual human subject and in the monkey with receptor blockade decreased exposure to organs with high PBR densities and enhanced uptake in excretory and metabolic pathways.     

(11)C]DAA1106: radiosynthesis and in vivo binding to peripheral benzodiazepine receptors in mouse brain

DAA1106 (N-(2,5-Dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide), is a potent and selective ligand for peripheral benzodiazepine receptors (PBR) in mitochondrial fractions of rat (K(i)=0.043 nM) and monkey (K(i)=0.188 nM) brains. This compound was labeled by [(11)C]methylation of a corresponding desmethyl precursor (DAA1123) with [(11)C]CH(3)I in the presence of NaH, with a 72+/-16% (corrected for decay) incorporation yield of radioactivity. After HPLC purification, [(11)C]DAA1106 was obtained with > or =98% radiochemical purity and specific activity of 90-156 GBq/micromol at the end of synthesis. After iv injection of [(11)C]DAA1106 into mice, high accumulations of radioactivity were found in the olfactory bulb and cerebellum, the high PBR density regions in the brain. Coinjection of [(11)C]DAA1106 with unlabeled DAA1106 and PBR-selective PK11195 displayed a significant reduction of radioactivity, suggesting a high specific binding of [(11)C]DAA1106 to PBR. Although this tracer was rapidly metabolized in the plasma, only [(11)C]DAA1106 was detected in the brain tissues, suggesting the specific binding in the brain due to the tracer itself. These findings revealed that [(11)C]DAA1106 is a potential and selective positron emitting radioligand for PBR.