In vitro and in vivo characterization of (+)-3-[11C]cyano-dizocilpine

Summary. (+)-3-[¹¹C]Cyano-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine ([¹¹C]MKC) was successfully synthesized as a potential radiotracer for PET studies on the NMDA receptor channel complex. In vitro binding properties of [¹¹C]MKC were investigated with newly developed techniques for efficient evaluation of ¹¹C-labeled compounds. The association curve of [¹¹C]MKC binding in rat forebrain membranes showed that the specific binding reached an equilibrium within 30 min. Specific binding was saturable with affinity constant K_D = 8.2 ± 0.4 nM and B_max = 1.62 ± 0.04 pmol/mg protein with glutamate and glycine included in the incubation medium. The binding of [¹¹C]MKC was decreased by extensive washing of the membrane preparation. (+)- and (−)-Dizocilpine, 3-cyano-dizocilpine, and ketamine inhibited the specific binding of [¹¹C]MKC with IC₅₀ values of 37.3, 445.0, 65.8 nM and 3.91 μM, respectively. High specific binding in in vitro autoradiography was distributed predominantly in telencephalic regions (the hippocampus, cerebral cortex, and striatum) followed by thalamus. PET studies using rhesus monkeys under anesthesia showed high uptake of [¹¹C]MKC in the temporoparietal and frontal cerebral cortices, striatum, and thalamic regions, although it is problematic to verify the specific binding in vivo by PET. Accepted January 5, 1998; received October 7, 1997     

Brain PET measurement of PDE10A occupancy by TAK‐063, a new PDE10A inhibitor,using [11C]T‐773 in nonhuman primates

Because phosphodiesterase 10A (PDE10A) degrades both cyclic adenosine monophosphate and cyclic guanosine monophosphate and is distributed mainly in the striatum, PDE10A inhibitors have been considered to potentially be useful therapeutic agents for psychiatric and neurodegenerative diseases such as schizophrenia and Huntington's disease. We measured striatal PDE10A occupancy by TAK‐063, a newly developed compound with high affinity and selectivity for PDE10A, using PET with [¹¹C]T‐773 in nonhuman primates. Two 123‐min dynamic PET measurements were performed on three female rhesus monkeys, once at baseline and again after intravenous administration of different doses of TAK‐063 (0.2–1.6 mg/kg). Total distribution volume (V_T) was calculated with a two‐tissue compartment model using metabolite‐corrected plasma input. Although the in vitro autoradiography did not show high specific binding to [¹¹C]T‐773 in the cerebellum, V_T in the cerebellum decreased after TAK‐063 treatment. The specific binding to PDE10A (V_S) was calculated as the difference of the V_T between the target regions and the cerebellum. PDE10A occupancy was calculated as the percent change of V_S. The average PDE10A occupancy of the caudate nucleus and putamen was 35.2% at 0.2 mg/kg and 83.2% at 1.6 mg/kg. In conclusion, this nonhuman primate PET study demonstrated that [¹¹C]T‐773 is useful to estimate the PDE10A occupancy by TAK‐063 in the striatum although there is in vivo interaction of the uptake between [¹¹C]T‐773 and TAK‐063 in the cerebellum. These results warrant further clinical occupancy study for TAK‐063. Synapse 70:253–263, 2016 . © 2016 Wiley Periodicals, Inc.     

在转基因痴呆小鼠及猕猴[~(11)C]MP4A PET显像脑乙酰胆碱酯酶活性

目的:应用PET诊断显像剂[~(11)C]4-乙酰氧基-N-甲基哌啶([~(11)C]MP4A)对转基因痴呆小鼠、老龄小鼠及猕猴进行活体脑乙酰胆碱酯酶(AChE)活性PET显像研究。方法:小鼠经静脉注射[~(11)C]MP4A后进行PET/CT扫描,采集图像测量并计算出各感兴趣区域放射总量百分比(%ID),分析两组小鼠间及猕猴的PET显像差异与特点。扫描后即刻分离小鼠大脑、小脑和脑干,称重并测定~(11)C计数,计算放射性摄取率(%ID/g),比较两组小鼠脑放射性摄取率分布差异。结果:两组小鼠放射性显像清晰,脑放射性聚集对称。转基因痴呆鼠脑中放射性聚集较老龄鼠稀疏。转基因小鼠和老龄鼠大脑平均放射性摄取率值分别为1.49%ID/g和2.62%ID/g,差异有统计学意义(P=0.025)。猕猴脑内放射性显像清晰,放射性物质对称浓聚于皮质下,皮质聚集中等。结论:小鼠和正常猕猴[~(11)C]MP4A PET显像清晰,放射性物质对称分布于皮质下,[~(11)C]MP4A可以活体显像脑AChE的活性。     

Further evaluation of [11C]MP‐10 as a radiotracer for phosphodiesterase 10A: PET imaging study in rhesus monkeys and brain tissue metabolite analysis

[¹¹C]MP‐10 is a potent and specific PET tracer previously shown to be suitable for imaging the phosphodiesterase 10A (PDE10A) in baboons with reversible kinetics and high specific binding. However, another report indicated that [¹¹C]MP‐10 displayed seemingly irreversible kinetics in rhesus monkeys, potentially due to the presence of a radiolabeled metabolite capable of penetrating the blood‐brain‐barrier (BBB) into the brain. This study was designed to address the discrepancies between the species by re‐evaluating [¹¹C]MP‐10 in vivo in rhesus monkey with baseline scans to assess tissue uptake kinetics and self‐blocking scans with unlabeled MP‐10 to determine binding specificity. Ex vivo studies with one rhesus monkey and 4 Sprague‐Dawley rats were also performed to investigate the presence of radiolabeled metabolites in the brain. Our results indicated that [¹¹C]MP‐10 displayed reversible uptake kinetics in rhesus monkeys, albeit slower than in baboons. Administration of unlabeled MP‐10 reduced the binding of [¹¹C]MP‐10 in a dose‐dependent manner in all brain regions including the cerebellum. Consequently, the cerebellum appeared not to be a suitable reference tissue in rhesus monkeys. Regional volume of distribution (V_T) was mostly reliably derived with the multilinear analysis (MA1) method. In ex vivo studies in the monkey and rats only negligible amount of radiometabolites was seen in the brain of either species. In summary, results from the present study strongly support the suitability of [¹¹C]MP‐10 as a radiotracer for PET imaging and quantification of PDE10A in nonhuman primates. Synapse 69:86–95, 2015. © 2014 Wiley Periodicals, Inc.     

PET studies on the brain uptake and regional distribution of [11C]vinpocetine in human subjects

OBJECTIVES: Vinpocetine is a compound widely used in the prevention and treatment of cerebrovascular diseases. It is still not clear whether the drug has a direct and specific effect on neurotransmission or its effects are due to extracerebral actions, such as changes in cerebral blood flow. The main objective of the present investigation was to determine the global uptake and regional distribution of radiolabelled vinpocetine in the human brain in order to explore whether it may have direct central nervous system effects. MATERIAL AND METHODS: Three healthy subjects were examined with positron emission tomography and [11C]vinpocetine. The regional uptake was determined in anatomically defined volumes-of-interest. The fractions of [11C]vinpocetine and labelled metabolites in plasma were determined using high pressure liquid chromatography. RESULTS: The uptake of [11C]vinpocetine in brain was rapid and 3.7% (mean; n = 4) of the total radioactivity injected was in brain 2 min after radioligand administration. The uptake was heterogeneously distributed among brain regions. When compared with the cerebellum, an a priori reference region, the highest regional uptake was in the thalamus, upper brain stem, striatum and cortex. Following an initial peak, the total concentration of radioactivity in blood was relatively stable with time, whereas the concentration of the unchanged compound decreased with time in an exponential manner. CONCLUSION: Vinpocetine, administered intravenously in humans, readily passes the blood-brain barrier and enters the brain. Its regional uptake and distribution in the brain is heterogeneous, indicating binding to specific sites. The brain regions showing increased uptake in the human brain correspond to those in which vinpocetine has been shown to induce elevated metabolism and blood flow. These observations support the hypothesis that vinpocetine has direct neuronal actions in the human brain.     

Development of novel PET probe [11C](R,R)HAPT and its stereoisomer [11C](S,S)HAPT for vesicular acetylcholine transporter imaging: A PET study in conscious monkey

Carbon‐11‐labeled (R,R)trans‐8‐methyl‐2‐hydroxy‐3‐[4‐[2‐aminophenyl]piperizinyl]‐tetralin ([¹¹C](R,R)HAPT) and its stereoisomer [¹¹C](S,S)HAPT were developed for imaging vesicular acetylcholine transporters (VAChTs), exclusively located in presynaptic cholinergic neurons. Both positron emission tomography (PET) probes were evaluated in the brain of conscious monkey (Macaca mulatta) using high‐resolution PET. Time‐activity curves (TACs) of [¹¹C](R,R)HAPT peaked within 5 min after the injection in all regions except the caudate and putamen, both of which showed peaks around 20 min postinjection. The regional distribution patterns of [¹¹C](R,R)HAPT determined as total distribution volume (V_t) were highest in the putamen, high in the caudate, intermediate in the amygdala, hippocampus, and thalamus, lower in the cingulate gyrus and frontal, temporal, and occipital cortices, and lowest in the cerebellum. In contrast, the distribution and TACs of [¹¹C](S,S)HAPT were homogeneous in all regions. The uptake of [¹¹C](R,R)HAPT was reduced by 1 mg/kg (?)‐vesamicol, a specific VAChT antagonist, in all regions except the cerebellum, but not by 0.1 mg/kg SA4503, a specific sigma‐1 receptor agonist. These results well reflect the in vitro affinity assessments using rat cerebral membranes. They also demonstrate that [¹¹C](R,R)HAPT is a potential PET probe for noninvasive and quantitative imaging of VAChT in the living brain. Synapse 68:283–292, 2014 . © 2014 Wiley Periodicals, Inc.     

Within-subject comparison of [11C]-(+)-PHNO and [11C]raclopride sensitivity to acute amphetamine challenge in healthy humans

[¹¹C]PHNO is a D₂/D₃ agonist positron emission tomography radiotracer, with higher in vivo affinity for D₃ than for D₂ receptors. As [¹¹C]-(+)-PHNO is an agonist, its in vivo binding is expected to be more affected by acute fluctuations in synaptic dopamine than that of antagonist radiotracers such as [¹¹C]raclopride. In this study, the authors compared the effects of an oral dose of the dopamine releaser amphetamine (0.3 mg/kg) on in vivo binding of [¹¹C]-(+)-PHNO and [¹¹C]raclopride in healthy subjects, using a within-subjects, counterbalanced, open-label design. In the dorsal striatum, where the density of D₃ receptors is negligible and both tracers predominantly bind to D₂ receptors, the reduction of [¹¹C]-(+)-PHNO binding potential (BP_ND) was 1.5 times larger than that of [¹¹C]raclopride. The gain in sensitivity associated with the agonist [¹¹C]-(+)-PHNO implies that ∼65% of D₂ receptors are in the high-affinity state in vivo. In extrastriatal regions, where [¹¹C]-(+)-PHNO predominantly binds to D₃ receptors, the amphetamine effect on [¹¹C]-(+)-PHNO BP_ND was even larger, consistent with the higher affinity of dopamine for D₃. This study indicates that [¹¹C]-(+)-PHNO is superior to [¹¹C]raclopride for studying acute fluctuations in synaptic dopamine in the human striatum. [¹¹C]-(+)-PHNO also enables measurement of synaptic dopamine in D₃ regions.     

Effect on [11C]DASB binding after tranylcypromine-induced increase in serotonin concentration: positron emission tomography studies in monkeys and rats

Several research groups have demonstrated that under specific conditions, in vivo neuroreceptor binding techniques can be used to measure acute changes in the concentrations of endogenous transmitters in the vicinity of neuroreceptors. The aim of this study was to investigate whether [(11)C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile ([(11)C]DASB) binding to the plasma membrane serotonin transporter (SERT) in the rhesus monkey and rat brain decreased after a pharmacologically-induced increase in the interstitial serotonin (5HT) concentration. Three rhesus monkeys were given repeated single boluses of [(11)C]DASB in sequential positron emission tomography (PET) experiments. Rats were given the tracer as a bolus dose plus a constant infusion. In vivo binding in both models was studied before and after presumably having increased interstitial 5HT concentrations using tranylcypromine (TCP), which inhibits the enzyme (monoamine oxidase, MAO), that degrades 5HT. The rat brain tissue was analyzed using high-performance liquid chromatography (HPLC) to determine the proportion of the PET signal comprising unchanged [(11)C]DASB. The binding of [(11)C]DASB in the thalamus decreased in both rhesus monkeys and rats after TCP administration. The possibility of using [(11)C]DASB as a tool for monitoring changes in endogenous serotonin concentrations merits further investigation.     

In vivo imaging of microglial activation with [11C](R)-PK11195 PET in corticobasal degeneration.

Corticobasal degeneration (CBD) is a neurodegenerative parkinsonian disorder of unknown cause that shows considerable clinical heterogeneity. In CBD, activated microglia have been shown to be associated closely with the extensive tau pathology found in the affected basal ganglia, brainstem nuclei, and cortical regions. We report on the use of [(11)C](R)-(1-[2-chlorophenyl]-N-methyl-N-[1-methylpropyl]-3-isoquinoline carboxamide) (PK11195) positron emission tomography (PET), a marker of peripheral benzodiazepine binding sites (PBBS) that are expressed by activated microglia, to demonstrate in vivo the degree and distribution of glial response to the degenerative process in 4 patients with CBD. Compared with normal age-matched controls, the CBD patient group showed significantly increased mean [(11)C](R)-PK11195 binding in the caudate nucleus, putamen, substantia nigra, pons, pre- and postcentral gyrus, and the frontal lobe. [11C](R)-PK11195 PET reveals a pattern of increased microglial activation in CBD patients involving cortical regions and the basal ganglia that corresponds well with the known distribution of neuropathological changes, which may therefore help to characterize in vivo the underlying disease activity in CBD.     

PDE4 inhibition enhances hippocampal synaptic plasticity in vivo and rescues MK801-induced impairment of long-term potentiation and object recognition memory in an animal model of psychosis

Inhibition of phosphodiesterase type 4 (PDE4) by rolipram (4-(3-(cyclopentyloxy)-4-methoxyphenyl)-pyrrolidin-2-one) has been the focus of many behavioral and molecular studies in the recent years. Rolipram exhibits memory-enhancing effects in rodents. In vitro studies have shown that long-term potentiation (LTP), which may comprise a cellular substrate for learning, is also enhanced by rolipram. However, effects have not been assessed in vivo. Rolipram has antipsychotic properties. Psychosis affects cognition and in animal models of psychosis LTP is impaired. In this study, we investigated if PDE4 inhibition improves LTP in healthy animals in vivo and if PDE4 inhibition rescues impaired LTP and prevents object recognition memory deficits in an animal model of psychosis. Recordings were made from the hippocampus of adult, freely behaving Wistar rats. Thirty minutes after treatment with rolipram or vehicle, a tetanus was applied to the medial perforant path to elicit short-term potentiation (STP) in the dentate gyrus. At this time-point, radioimmunoassay revealed that rolipram significantly elevated cyclic adenosine monophosphate levels in the dorsal hippocampus, in line with reports by others that rolipram mediates decreased PDE4 activity. In healthy animals, both intracerebroventricular and subcutaneous treatment with rolipram facilitated STP into LTP, suggesting that PDE4 inhibition may have a permissive role in plasticity mechanisms that are relevant for learning and memory. One week after a single systemic treatment with the irreversible N-methyl--aspartate antagonist, MK801, LTP and object recognition memory were significantly impaired, but could be rescued by PDE4 inhibition. These data suggest that the relief of cognitive disturbances in psychosis models by rolipram may be mediated in part by a rescue of hippocampal LTP.     

In vivo imaging of microglial activation with [11C](R)-PK11195 PET in progressive supranuclear palsy.

Progressive supranuclear palsy (PSP) is a neurodegenerative disease presenting with voluntary gaze difficulties, early falls, and Parkinsonism. Neuronal loss, associated with intracellular neurofibrillary tangles and activated microglia, is found targeting the basal ganglia, brainstem nuclei, and frontal cortex. [11C](R)-PK11195 PET is a marker of peripheral benzodiazepine binding sites (PBBS) expressed by activated microglia. We have used [11C](R)-PK11195 PET to demonstrate in vivo the degree and distribution of the glial response to the degenerative process in four patients with PSP. Compared to normal age-matched controls, the PSP patient group showed significantly increased mean [11C](R)-PK11195 binding in the basal ganglia, midbrain, the frontal lobe, and the cerebellum. Two of the patients were rescanned after 6 to 10 months and during that time the level of microglial activation remained stable. [11C](R)-PK11195 PET reveals a pattern of increased microglial activation in PSP patients involving cortical and subcortical regions that corresponds well with the known distribution of neuropathological changes. [11C](R)-PK11195 PET, therefore, may help in characterizing in vivo the underlying disease activity in PSP.     

Evaluation in monkey of two candidate PET radioligands, [11C]RX‐1 and [18F]RX‐2, for imaging brain 5‐HT4 receptors

The serotonin subtype‐4 (5‐HT₄) receptor, which is known to be involved physiologically in learning and memory, and pathologically in Alzheimer's disease, anxiety, and other neuropsychiatric disorders—has few radioligands readily available for imaging in vivo. We have previously reported two novel 5‐HT₄ receptor radioligands, namely [methoxy‐¹¹C](1‐butylpiperidin‐4‐yl)methyl 4‐amino‐3‐methoxybenzoate; [¹¹C]RX‐1), and the [¹⁸F]3‐fluoromethoxy analog ([¹⁸F]RX‐2), and in this study we evaluated them by PET in rhesus monkey. Brain scans were performed at baseline, receptor preblock or displacement conditions using SB 207710, a 5‐HT₄ receptor antagonist, on the same day for [¹¹C]RX‐1 and on different days for [¹⁸F]RX‐2. Specific‐to‐nondisplaceable ratio (BP_ND) was measured with the simplified reference tissue model from all baseline scans. To determine specific binding, total distribution volume (V_T) was also measured in some monkeys by radiometabolite‐corrected arterial input function after ex vivo inhibition of esterases from baseline and blocked scans. Both radioligands showed moderate to high peak brain uptake of radioactivity (2–6 SUV). Regional BP_ND values were in the rank order of known 5‐HT₄ receptor distribution with a trend for higher BP_ND values from [¹⁸F]RX‐2. One‐tissue compartmental model provided good fits with well identified V_T values for both radioligands. In the highest 5‐HT₄ receptor density region, striatum, 50–60% of total binding was specific. The V_T in receptor‐poor cerebellum reached stable values by about 60 min for both radioligands indicating little influence of radiometabolites on brain signal. In conclusion, both [¹¹C]RX‐1 and [¹⁸F]RX‐2 showed positive attributes for PET imaging of brain 5‐HT₄ receptors, validating the radioligand design strategy. Synapse 68:613–623, 2014 . © 2014 Wiley Periodicals, Inc.     

Evaluation of [4‐O‐methyl‐11C]KW‐6002 as a potential PET ligand for mapping central adenosine A2A receptors in rats

KW‐6002, a xanthine‐based adenosine A_2A antagonist, was labelled with the positron emitter carbon‐11 by O‐methylation of its precursor, KF23325, using [¹¹C]iodomethane and was evaluated in rats as a putative in vivo radioligand for positron emission tomography (PET). Following intravenous injection of [¹¹C]KW‐6002, radioactivity was measured in blood, plasma, peripheral tissues, and in discrete brain tissues over a 2‐h time period commensurate with PET scanning. In brain, [¹¹C]KW‐6002 showed highest retention in striata, with evidence of saturable binding, and lowest retention in frontal cortex (a tissue low in adenosine A_2A receptors). PET scanning with [¹¹C]KW‐6002 demonstrated a specific signal in the striata which could be described using compartmental modelling. Specific binding was, however, also detected in extrastriatal regions, including brain areas reported to have low adenosine A_2A receptor density. Blocking studies with the A₁ selective antagonist KF15372 and the non xanthine‐type A_2A antagonist ZM 241385 failed to elucidate the nature of this binding. Thus, although [¹¹C]KW‐6002 shows some potential for development as a PET ligand for quantifying striatal adenosine A_2A receptor function, its in vivo selectivity requires further investigation. Synapse 42:164–176, 2001. © 2001 Wiley‐Liss, Inc.     

(11)C]MADAM, a new serotonin transporter radioligand characterized in the monkey brain by PET

The aim of this study was to explore the potential of a new selective serotonin transporter (5-HTT) inhibitor, N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine (MADAM, K(i)=1.65 nM), as a PET radioligand for examination of 5-HTT in the nonhuman primate brain. MADAM was radiolabeled by an N-methylation reaction using [(11)C]methyl triflate and the binding was characterized by PET in four cynomolgus monkeys. Metabolite levels in plasma were measured by gradient high-performance liquid chromatography (HPLC). The radiochemical incorporation yield of [(11)C]MADAM was 75-80% and the specific radioactivity at the time of administration was 34-652 GBq/micromol (n=8). The highest uptake of radioactivity was observed in striatum, thalamus, mesencephalon, and the lower brainstem. Lower binding was detected in neocortex and the lowest radioactive uptake was found in the cerebellum. This distribution is in accordance with the known expression of 5-HTT in vitro. The fraction of the total radioactivity in monkey plasma representing unchanged [(11)C]MADAM was 20% at 45 min after injection, as measured by gradient HPLC. Pretreatment measurements, using unlabeled citalopram, GBR 12909, and maprotiline, as well as a displacement measurement, using unlabeled MADAM, confirmed that [(11)C]MADAM binds selectively and reversibly to 5-HTT, and support the use of the cerebellum as reference region. The present characterization of binding in the monkey brain suggests that [(11)C]MADAM is a potential PET radioligand for quantitative studies of 5-HTT binding in the human brain.     

Differential effects of stress on [11C]raclopride and [11C]MNPA binding to striatal D2/D3 dopamine receptors: A PET study in conscious monkeys

It has been reported that stress and facilitation of dopamine neuronal system are closely related. In the present study, the effects of stress on the binding of antagonist‐based [¹¹C]raclopride and agonist‐based (R)‐2‐CH3O‐N‐n‐ propylnorapomorphine ([¹¹C]MNPA) to D₂/D₃ receptors were evaluated in the striatum of conscious monkey brain. The stress state assessed from plasma cortisol level was negatively correlated with [¹¹C]raclopride binding as expected. It was noteworthy that [¹¹C]MNPA binding exhibited a positive correlation with stress state; thus, the animals with higher cortisol levels showed higher binding to D₂/D₃ receptors. Synapse 65:84–89, 2011. © 2010 Wiley‐Liss, Inc.     

Ex vivo and in vivo evaluation of (2R,3R)-5-[(18)F]-fluoroethoxy- and fluoropropoxy-benzovesamicol, as PET radioligands for the vesicular acetylcholine transporter

Molecular imaging of the vesicular acetylcholine transporter (VAChT) using positron emission tomography (PET) may provide insights into early diagnosis and better understanding of Alzheimer's disease. We further characterized the VAChT ligand (2R,3R)-5-FEOBV (1) and developed new fluoropropoxy analogues. Ex vivo studies of the new nonradiolabeled analogues (2R,3R)-5-FPOBV (2) (k(D) = 0.7 nM) and (2S,3S)-5-FPOBV (3) (k(D) = 8.8 nM) were performed in rat brain and showed an enantioselective inhibition of (-)-5-[(125)I]-IBVM uptake in striatum, cortex, and hippocampus (e.g., 74% for 2 and only 54% for 3 in the cortex). Radiochemical procedures were developed to produce [(18)F]1 and [(18)F]2 as potential imaging agent for the VAChT. The radiochemistry was carried out in a one step procedure, with radiolabeling yields of 17 and 2.6% (range: 1-5.4), respectively, nondecay corrected with good specific activity: 124-338 GBq/micromol. The radiochemical purity was greater than 98%. The biological (ex vivo and in vivo) properties of these radioligands were evaluated in rats and showed a low (less then 0.1% of the injected dose) and homogeneous brain uptake. The in vivo PET study of [(18)F]2 performed in baboon also revealed rapid defluorination as the main problem. Therefore [(18)F]1 and [(18)F]2 appear to be unsuitable for in vivo imaging of the VAChT using PET.     

In vivo imaging of microglial activation by positron emission tomography with [11C]PBR28 in the 5XFAD model of Alzheimer's disease

Microglial activation has been linked with deficits in neuronal function and synaptic plasticity in Alzheimer's disease (AD). The mitochondrial translocator protein (TSPO) is known to be upregulated in reactive microglia. Accurate visualization and quantification of microglial density by PET imaging using the TSPO tracer [¹¹C]‐R‐PK11195 has been challenging due to the limitations of the ligand. In this study, it was aimed to evaluate the new TSPO tracer [¹¹C]PBR28 as a marker for microglial activation in the 5XFAD transgenic mouse model of AD. Dynamic PET scans were acquired following intravenous administration of [¹¹C]PBR28 in 6‐month‐old 5XFAD mice and in wild‐type controls. Autoradiography with [³H]PBR28 was carried out in the same brains to further confirm the distribution of the radioligand. In addition, immunohistochemistry was performed on adjacent brain sections of the same mice to evaluate the co‐localization of TSPO with microglia. PET imaging revealed that brain uptake of [¹¹C]PBR28 in 5XFAD mice was increased compared with control mice. Moreover, binding of [³H]PBR28, measured by autoradiography, was enriched in cortical and hippocampal brain regions, coinciding with the positive staining of the microglial marker Iba‐1 and amyloid deposits in the same areas. Furthermore, double‐staining using antibodies against TSPO demonstrated co‐localization of TSPO with microglia and not with astrocytes in 5XFAD mice and human post‐mortem AD brains. The data provided support of the suitability of [¹¹C]PBR28 as a tool for in vivo monitoring of microglial activation and assessment of treatment response in future studies using animal models of AD. GLIA 2016;64:993–1006     

Microglia, amyloid, and cognition in Alzheimer's disease: An [11C](R)PK11195-PET and [11C]PIB-PET study

[11C](R)PK11195-PET is a marker of activated microglia while [11C]PIB-PET detects raised amyloid load. Here we studied in vivo the distributions of amyloid load and microglial activation in Alzheimer's disease (AD) and their relationship with cognitive status. Thirteen AD subjects had [11C](R)PK11195-PET and [11C]PIB-PET scans. Ten healthy controls had [11C](R)PK11195-PET and 14 controls had [11C]PIB-PET scans. Region-of-interest analysis of [11C](R)PK11195-PET detected significant 20–35% increases in microglial activation in frontal, temporal, parietal, occipital and cingulate cortices (p < 0.05) of the AD subjects. [11C]PIB-PET revealed significant two-fold increases in amyloid load in these same cortical areas (p < 0.0001) and SPM (statistical parametric mapping) analysis confirmed the localisation of these increases to association areas. MMSE scores in AD subjects correlated with levels of cortical microglial activation but not with amyloid load. The inverse correlation between MMSE and microglial activation is compatible with a role of microglia in neuronal damage.