Binding Of Tracers Research Articles

Mechanistic Insights into the Binding of Different Positron Emission Tomography Tracers to Chronic Traumatic Encephalopathy Tau Protofibrils.

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease associated with exposure to repetitive head impacts, and it is neuropathologically defined as the accumulation of abnormally hyperphosphorylated tau (p-tau). Early detection of p-tau in the brain is of great value in the prevention and treatment of CTE. Previous experimental studies reported that positron emission tomography (PET) technique using several tau tracers are available for imaging certain neurodegenerative diseases. However, few studies have focused on the development of CTE tau tracers. In this work, we performed conventional molecular docking and molecular dynamics simulations to address the binding properties and mechanisms of PET tracers (18F-PM-PBB3, 18F-CBD-2115, 18F-PI-2620, 18F-RO-948, 18F-MK-6240, and 18F-flortaucipir) to CTE tau protofibrils. The results show that the hydrophobic cavity and the top of the concave structure of CTE tau protofibrils are the preferred binding sites for the six tracers, and 18F-PM-PBB3 has the most competitive binding affinity to CTE tau protofibrils. Further investigation into the binding patterns of the six tracers to the CTE tau protofibrils showed that 18F-CBD-2115 and 18F-PM-PBB3 have a high number of H-bonds and hydrophobic contacts with tau protofibrils, resulting in strong hydrogen bonding and hydrophobic interactions; 18F-flortaucipir/18F-PI-2620 and 18F-PI-2620/18F-RO-948 form more intense π-π and cation-π interactions with tau protofibrils, respectively. Subsequently, we conducted a detailed analysis of the binding mechanism of 18F-PM-PBB3 to CTE tau protofibrils. The benzothiazole ring of 18F-PM-PBB3 exhibits stronger π-π stacking and cation-π interactions with tau protofibrils than the pyridine ring and forms a more concentrated T-shaped π-π stacking pattern. This study contributes to understanding the binding mechanism of PET tracers to CTE tau protofibrils and provides new insights into the design of potential novel tracers.

Tau PET imaging in progressive supranuclear palsy: a systematic review and meta-analysis.

To evaluate the difference of tau burden between patients with progressive supranuclear palsy (PSP) and healthy controls (HCs) or other neurodegenerative diseases using tau-positron emission tomography (PET) imaging. A systematic search on PubMed, Embase, and Web of Science databases was performed for tau-PET studies in PSP patients, up to April 1, 2022. Standardized mean differences (SMDs) of tau tracer uptake were calculated using random-effects models. Subgroup analysis based on the type of tau tracers, meta-regression, and sensitivity analysis were conducted. Twenty-seven studies comprising 553 PSP, 626 HCs, and 406 other neurodegenerative diseases were included. Compared with HCs, PSP patients showed elevated tau binding in basal ganglia, midbrain, dentate nucleus, cerebellar white matter, and frontal lobe with decreasing SMD (SMD: 0.390-1.698). Compared with Parkinson's disease patients, increased tau binding was identified in the midbrain, basal ganglia, dentate nucleus, and frontal and parietal lobe in PSP patients with decreasing SMD (SMD: 0.503-1.853). PSP patients showed higher tau binding in the subthalamic nucleus (SMD = 1.351) and globus pallidus (SMD = 1.000), and lower binding in the cortex and parahippocampal gyrus than Alzheimer's disease patients (SMD: -2.976 to -1.018). PSP patients showed higher midbrain tau binding than multiple system atrophy patients (SMD = 1.269). Tau PET imaging indicates different topography of tau deposition between PSP patients and HCs or other neurodegenerative disorders. The affinity and selectivity of tracers for 4R-tau and the off-target binding of tracers should be considered when interpreting the results.

Patient-derived parathyroid organoids as a tracer and drug-screening application model.

Parathyroid diseases are characterized by dysregulation of calcium homeostasis and alterations in parathyroid hormone (PTH) excretion. The development of parathyroid-targeted treatment and imaging tracers could benefit from invitro models. Therefore, we aim to establish a patient-derived parathyroid organoid model representing human parathyroid tissue. Hyperplastic parathyroid tissue was dispersed, and parathyroid organoids (PTOs) were cultured and characterized. PTO-derived cells exhibited self-renewal over several passages, indicative of the presence of putative stem cells. Immunofluorescence and RNA sequencing confirmed that PTOs phenocopy hyperplastic parathyroid tissue. Exposure of PTOs to increasing calcium concentrations and PTH-lowering drugs resulted in significantly reduced PTH excretion. PTOs showed specific binding of the imaging tracers 11C-methionine and 99mTc-sestamibi. These data show the functionality of PTOs resembling the parathyroid. This PTO model recapitulates the originating tissue on gene and protein expression and functionality, paving the way for future physiology studies and therapeutic target and tracer discovery.

The trends and significance of SSTR PET/CT added to MRI in follow-up imaging of low-grade meningioma treated with fractionated proton therapy.

Overexpression of the somatostatin receptor (SSTR) has led to adoption of SSTR PET/CT for diagnosis and radiotherapy planning in meningioma, but data on SSTR expression during follow-up remain scarce. We investigated PET/CT quantifiers of SSTR tracers in WHO gradeI meningioma following fractionated proton beam therapy (PBT) compared to standard response assessment with MRI. Twenty-two patients diagnosed with low-grade meningioma treated by PBT were included. Follow-up included clinical visits, MRI, and [68Ga]Ga-DOTATOC PET/CT scans. Radiologic tumor response, MRI and PET volume (VMRI and VPET), maximum and mean standardied uptake value (SUVmax/SUVmean), total lesion activity (TLA), and heterogeneity index (HI) were evaluated. Median follow-up was 35.3months (range: 6.4-47.9). Nineteen patients (86.4%, p = 0.0009) showed adecrease of SUVmax between baseline and first follow-up PET/CT (median: -24%, range: -53% to +89%) and in 81.8% of all cases, the SUVmax, SUVmean, and TLA at last follow-up were eventually lower than at baseline (p = 0.0043). Ambiguous trends without significance between the timepoints analyzed were observed for VPET. HI increased between baseline and last follow-up in 75% of cases (p = 0.024). All patients remained radiologically and clinically stable. Median VMRI decreased by -9.3% (range 0-32.5%, p < 0.0001) between baseline and last follow-up. PET/CT in follow-up of irradiated meningioma showed an early trend towards decreased binding of SSTR-specific tracers following radiation and MRI demonstrated consistently stable or decreasing tumor volume. Translational research is needed to clarify the underlying biology of the subsequent increase in SSTR PET quantifiers.

Cardiac microcalcifications in transthyretin (ATTR) amyloidosis

BackgroundBone tracers bind to amyloid-containing heart of most patients with ATTR amyloidosis. Amyloid deposits outside the heart are often scarce and bone scintigraphy is increasingly often used to diagnose cardiac involvement. However, the nature of the binding of bone tracers to the heart is not clear. ObjectiveTo identify possible calcium deposits in hearts with amyloid, explaining bone tracer binding. Methods and resultsFormalin-fixed and paraffin embedded cardiac specimens from three patients with ATTR and one with AL amyloidosis, all with cardiac deposits, were studied. The specimens covered large parts of the heart. Sections were stained immunohistochemically for ATTR deposits and according to von Kóssa for calcifications. The study identified in all hearts, but particularly in the ATTR materials, focal, tight swarms of tiny calcifications. These were sometimes associated with amyloid but found as frequent in areas without such deposits. Autoradiography with [99mTc]Tc labelled 3,3-disphos-phono-1,2-propanodicarboxylic acid (DPD) revealed labelling in von Kóssa positive areas. Electron microscopically the particles were not amorphous but had a complex structured appearance and were often surrounded by a membrane, indicating a cellular origin. Labelling with antibodies against ubiquitin and P62 pointed to result from autophagy. ConclusionsOur study indicates that binding of skeletal probes to amyloid-containing hearts depends on an irregular presence of clouds of very tiny calcifications, which seem not to be directly associated with amyloid fibrils. Therefore, [99mTc]Tc-DPD bone scans can be considered surrogate markers of ATTR amyloid but have to be used carefully to estimate amyloid amount or disease progression.

Comparative preclinical evaluation of a microtubule PET tracer with a beta‐amyloid PET tracer in AD mice

AbstractBackgroundCurrently available PET tracers used in Alzheimer's disease (AD) have limited utility in prognosis and in revealing reliable clinicopathologic correlations of disease. Therefore, identification and validation of novel biomarkers for AD progression and preclinical disease onset through noninvasive neuroimaging is a top priority in biomedical research. We proposed to target microtubules (MTs) as an imaging target for AD in preclinical, postmortem and clinical studies that support the implication of altered regulation of MTs in AD and positively correlated with neurodegeneration process. Furthermore, MT loss in AD is contributed to a large number of mutated proteins, enzymes, and post‐translational modifications compared to other imaging targets. Therefore, targeting brain MTs may be advantageous, as cumulative loss of MTs is the final common pathway for a variety of biochemical pathologies leading to neurodegeneration in AD. [11C]MPC‐6827 is the pioneer BBB‐penetrating PET tracer available for in vivo imaging of MTs in brain.1,2 Herein, we report the comparative binding of [11C]MPC‐6827, and [11C]PiB in Aβ over expressing APP mutated transgenic J20 mice and controls.Method[11C]MPC‐6827 and [11C]PiB were synthesized using GE Tracerlab FX2 MEI and FX2N modules. Dynamic PET scans were performed for 30 min after tail vein injection of [11C]MPC‐6827 and [11C]PiB in 6‐month‐old J20 mice and littermates (n=4) using a Siemens Focus 220 microPET scanner. Image analyses were performed with Mango software on reconstructed data.ResultPET image analyses show ∼30% of reduced whole brain uptake of [11C]MPC‐6827 in J20 mice whereas, [11C]PiB exhibited modest higher binding (∼10%) in J20 than control mice. Standardized uptake values (SUVs) show similar trend of binding of tracers in prefrontal cortex (PFC) and hippocampus in J20 and control mice.ConclusionOur preliminary studies show that in J20 mice, binding of the [11C]MPC‐6827 is reduced in whole brain, hippocampus and PFC compared control mice and is inversely correlated with [11C]PiB binding. The effect size of [11C]MPC‐6827 seems higher than [11C]PiB. Therefore, [11C]MPC‐6827 could be used as a PET tracer for human brain imaging of AD and other NDs. Acknowledgement: This work was funded by Center for Biomedical Neuroscience (CBN) 2020‐2021, UT Health San Antonio, Texas, USA.

Regional binding of tau and amyloid PET tracers in Down syndrome autopsy brain tissue

IntroductionTau pathology is a major age-related event in Down syndrome with Alzheimer’s disease (DS-AD). Although recently, several different Tau PET tracers have been developed as biomarkers for AD, these tracers showed different binding properties in Alzheimer disease and other non-AD tauopathies. They have not been yet investigated in tissue obtained postmortem for DS-AD cases. Here, we evaluated the binding characteristics of two Tau PET tracers (3H-MK6240 and 3H-THK5117) and one amyloid (3H-PIB) ligand in the medial frontal gyrus (MFG) and hippocampus (HIPP) in tissue from adults with DS-AD and DS cases with mild cognitive impairment (MCI) compared to sporadic AD.MethodsTau and amyloid autoradiography were performed on paraffin-embedded sections. To confirm respective ligand targets, adjacent sections were immunoreacted for phospho-Tau (AT8) and stained for amyloid staining using Amylo-Glo.ResultsThe two Tau tracers showed a significant correlation with each other and with AT8, suggesting that both tracers were binding to Tau deposits. 3H-MK6240 Tau binding correlated with AT8 immunostaining but to a lesser degree than the 3H-THK5117 tracer, suggesting differences in binding sites between the two Tau tracers. 3H-THK5117, 3H-MK6240 and 3H-PIB displayed dense laminar binding in the HIPP and MFG in adult DS brains. A regional difference in Tau binding between adult DS and AD was observed suggesting differential regional Tau deposition in adult DS compared to AD, with higher THK binding density in the MFG in adult with DS compared to AD. No significant correlation was found between 3H-PIB and Amylo-Glo staining in adult DS brains suggesting that the amyloid PIB tracer binds to additional sites.ConclusionsThis study provides new insights into the regional binding distribution of a first-generation and a second-generation Tau tracer in limbic and neocortical regions in adults with DS, as well as regional differences in Tau binding in adult with DS vs. those with AD. These findings provide new information about the binding properties of two Tau radiotracers for the detection of Tau pathology in adults with DS in vivo and provide valuable data regarding Tau vs. amyloid binding in adult DS compared to AD.

Gold nanoparticle based double-labeling of melanoma extracellular vesicles to determine the specificity of uptake by cells and preferential accumulation in small metastatic lung tumors

BackgroundExtracellular vesicles (EVs) have shown great potential for targeted therapy, as they have a natural ability to pass through biological barriers and, depending on their origin, can preferentially accumulate at defined sites, including tumors. Analyzing the potential of EVs to target specific cells remains challenging, considering the unspecific binding of lipophilic tracers to other proteins, the limitations of fluorescence for deep tissue imaging and the effect of external labeling strategies on their natural tropism. In this work, we determined the cell-type specific tropism of B16F10-EVs towards cancer cell and metastatic tumors by using fluorescence analysis and quantitative gold labeling measurements. Surface functionalization of plasmonic gold nanoparticles was used to promote indirect labeling of EVs without affecting size distribution, polydispersity, surface charge, protein markers, cell uptake or in vivo biodistribution. Double-labeled EVs with gold and fluorescent dyes were injected into animals developing metastatic lung nodules and analyzed by fluorescence/computer tomography imaging, quantitative neutron activation analysis and gold-enhanced optical microscopy.ResultsWe determined that B16F10 cells preferentially take up their own EVs, when compared with colon adenocarcinoma, macrophage and kidney cell-derived EVs. In addition, we were able to detect the preferential accumulation of B16F10 EVs in small metastatic tumors located in lungs when compared with the rest of the organs, as well as their precise distribution between tumor vessels, alveolus and tumor nodules by histological analysis. Finally, we observed that tumor EVs can be used as effective vectors to increase gold nanoparticle delivery towards metastatic nodules.ConclusionsOur findings provide a valuable tool to study the distribution and interaction of EVs in mice and a novel strategy to improve the targeting of gold nanoparticles to cancer cells and metastatic nodules by using the natural properties of malignant EVs.

Cross-interaction of tau PET tracers with monoamine oxidase B: evidence from in silico modelling and in vivo imaging

PurposeSeveral tracers have been designed for tracking the abnormal accumulation of tau pathology in vivo. Recently, concerns have been raised about the sources of off-target binding for these tracers; inconclusive data propose binding for some tracers to monoamine oxidase B (MAO-B).MethodsMolecular docking and dynamics simulations were used to estimate the affinity and free energy for the binding of several tau tracers (FDDNP, THK523, THK5105, THK5317, THK5351, T807 [aka AV-1451, flortaucipir], T808, PBB3, RO-948, MK-6240, JNJ-311 and PI-2620) to MAO-B. These values were then compared with those for safinamide (MAO-B inhibitor). PET imaging was used with the tau tracer [18F]THK5317 and the MAO-B tracer [11C]DED in five patients with Alzheimer’s disease to investigate the MAO-B binding component of this first generation tau tracer in vivo.ResultsThe computational modelling studies identified a binding site for all the tau tracers on MAO-B; this was the same site as that for safinamide. The binding affinity and free energy of binding for the tau tracers to MAO-B was substantial and in a similar range to those for safinamide. The most recently developed tau tracers MK-6240, JNJ-311 and PI-2620 appeared, in silico, to have the lowest relative affinity for MAO-B. The in vivo investigations found that the regional distribution of binding for [18F]THK5317 was different from that for [11C]DED, although areas of suspected off-target [18F]THK5317 binding were detected. The binding relationship between [18F]THK5317 and [11C]DED depended on the availability of the MAO-B enzyme.ConclusionsThe developed tau tracers show in silico and in vivo evidence of cross-interaction with MAO-B; the MAO-B component of the tracer binding was dependent on the regional concentration of the enzyme.

Autoradiography validation of novel tau PET tracer [F-18]-MK-6240 on human postmortem brain tissue

[F-18]-MK-6240, a novel tau positron emission tomography (PET) tracer recently discovered for the in vivo detection of neurofibrillary tangles, has the potential to improve diagnostic accuracy in the detection of Alzheimer disease. We have examined regional and substrate-specific binding patterns as well as possible off-target binding of this tracer on human brain tissue to advance towards its validation. We applied [F-18]-MK-6240 phosphor screen and high resolution autoradiography to postmortem samples from patients with a definite pathological diagnosis of Alzheimer disease, frontotemporal lobar degeneration-tau (Pick’s disease, progressive supranuclear palsy and corticobasal degeneration), chronic traumatic encephalopathy, frontotemporal lobar degeneration-Tar DNA-binding protein 43 (TDP-43), dementia with Lewy bodies, cerebral amyloid angiopathy and elderly controls free of pathologic changes of neurodegenerative disease. We also directly compared the binding properties of [F-18]-MK-6240 and [F-18]-AV-1451 in human tissue, and examined potential nonspecific binding of both tau tracers to monoamine oxidases (MAO) by using autoradiography in the presence of selective monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) inhibitors. Our data indicate that MK-6240 strongly binds to neurofibrillary tangles in Alzheimer disease but does not seem to bind to a significant extent to tau aggregates in non-Alzheimer tauopathies, suggesting that it may have a limited utility for the in vivo detection of these pathologies. There is no evidence of binding to lesions containing β-amyloid, α-synuclein or TDP-43. In addition, we identified MK-6240 strong off-target binding to neuromelanin and melanin-containing cells, and some weaker binding to areas of hemorrhage. These binding patterns are nearly identical to those previously reported by our group and others for [F-18]-AV-1451. Of note, [F-18]-MK-6240 and [F-18]-AV-1451 autoradiographic binding signals were only weakly displaced by competing concentrations of selective MAO-B inhibitor deprenyl but not by MAO-A inhibitor clorgyline, suggesting that MAO enzymes do not appear to be a significant binding target of any of these two tracers. Together these novel findings provide relevant insights for the correct interpretation of in vivo [F-18]-MK-6240 PET imaging.

PET probes for imaging pancreatic islet cells

PET can detect very low concentrations of the probes at subpicomolar range in the target tissue with high sensitivity. The development of PET probes for imaging pancreatic islet cells depends on several key elements: the amount of target receptors in the beta cells; the target specificity and the target tissue delivery. This review summarized the latest developments of PET probes which are targeting proteins including GLP-1, VMAT2, GPR44, antigen, glucokinase and reporter genes in the beta cell for imaging pancreatic islet cells. A survey of the literature was performed to select the articles focusing on the development of PET probes for proteins targeting in the beta cell. Since PET probe development for imaging pancreatic islet cells is a narrow research field, the literature survey comprised of those articles published from 1993 when first probe C-11 DTBZ was synthesized to 2017 in English journals. In-111 SPECT probes are included for comparison study. A selection of 97 papers were identified following the literature review. The probes covered tritium-3-, carbon-11-, fluorine-18-, gallium-68-, copper-64- and indium-111 (for SPECT)-labeled radioligands. The PET imaging studies with the probes have been evaluated for in vitro, ex vivo, preclinical and clinical applications. Some issues of targeting probes have been addressed to develop ideal non-invasive PET probes for imaging pancreatic islet cells. From the literature, a number of new probes have been developed in recent years to improve their biological profile such as higher specific binding to target with lower nonspecific binding to surrounding tissue, or optimal residence time in the subjects. PET imaging modality for imaging pancreatic islet cell especially beta cell provided high resolution, high sensitivity, and accurate quantification through the biodistribution, pharmacokinetics and target binding of tracers. The demand for new PET probes for preclinical and clinical investigations is increasing. This review gives comprehensive overview of noteworthy probes which are proteins targeting in beta cell. The review provides some future research directions for this emerging field.

COMPARISON OF BINDING PROPERTIES OF THK5117, THK5351, PBB3 AND T807 IN AUTOPSIES OF ALZHEIMER DISEASE CASES

Several families of tau PET tracers recently developed have been studied in vivo in patients after having shown good in vitro binding-properties. Some differences in binding properties have been observed between the tau PET tracers in Alzheimer disease and non-AD tauopathies. Recent observations report presence of off-target binding. Here we aimed at characterizing the binding properties as well as possible off-target binding of the tau tracers by comparing two THK compounds, THK5117 and THK5351 with T807 and PBB3, head-to-head in the same human brain tissue, using autoradiography on large frozen section from AD cases and binding assay in brain homogenates. Autoradiographies on large frozen section were done using 3H-THK5351, 3H-THK5117, 11C-THK5351, 11C-PBB3 and 18F-T807. Competition autoradiography using unlabeled T807 were performed for the 11C compound. In vitro binding assay competition using 3H-THK5351 with unlabeled THK5351, THK5117 and T807 were performed in hippocampus homogenates. Competitions binding assay and autoradiography were performed using 3H-THK5117, 3H-L-deprenyl with unlabeled THK5117, R-deprenyl, rasagiline and chlorgyline to characterize the MAO off-target component. 11C-PPB3, 11C-THK5351 and 18F-T807 showed similar regional distribution in autoradiography but with different intensity. Unlabeled THK5351 displaced 50% of 11C-THK5351 binding and less than 10% of 11C-PBB3 binding in all regions. Unlabeled PBB3 displaced 50 % of 11C-PBB3 binding. Unlabeled T807 displaced up to 70 % of 18F-T807 binding in frontal and temporal cortex, 30% of 11C-THK5351 in frontal and temporal cortex and up to 40% in the basal ganglia, and displaced less than 20 %of 11C-PBB3 in all regions. In vitro binding assay using 3H-THK5351 in competition with unlabeled THK5351, THK5117 and T807 showed at least two binding sites. While a 4 times differences in affinity for the binding site in nanomolar range was observed between THK5351 and T807, they shared one binding site in the picomolar range. Tau tracers from three different families showed all different binding properties in AD cases. Greater similarity in binding properties was observed between THK5351 and T807 than with PBB3. These results may have an influence on the outcome of the clinical use in vivo PET study targeting different form of tauopathies.

Fluorinated PET Tracers for Molecular Imaging of σ1 Receptors in the Central Nervous System.

At first the role of σ1 receptors in various neurological, psychiatric and neurodegenerative disorders is discussed. In the second part, the principle of positron emission tomography (PET ) is described and the known fluorinated PET tracers for labeling of σ1 receptors are presented. The third part focuses on fluoroalkyl substituted spirocyclic PET tracers, which represent the most promising class of fluorinated PET tracers reported so far. The homologous fluoroalkyl derivatives 12-15 show high σ1 affinity (K i = 0.59-1.4 nM) and high selectivity over the σ2 subtype (408-1331-fold). The enantiomers of the fluoroethyl derivative fluspidine 13 were prepared and pharmacologically characterized. Whereas the (S)-configured enantiomer (S)-13 (K i = 2.3 nM) is 4-fold less active than the (R)-enantiomer (R)-13 (K i = 0.57 nM), (S)-13 is metabolically more stable. The interactions of (S)-13 and (R)-13 with the σ1 receptor were analyzed at the molecular level using the 3D homology model. In an automated radiosynthesis [18F](S)-13 and [18F](R)-13 were prepared by nucleophilic substitution of the tosylates (S)-17 and (R)-17 with K[18F]F in high radiochemical yield, high radiochemical purity and short reaction time. Application of both enantiomers [18F](S)-13 and [18F](R)-13 to mice and piglets led to fast uptake into the brain, but [18F](R)-13 did not show washout from the brain indicating a quasi-irreversible binding. Both radiotracers [18F](S)-13 and [18F](R)-13 were able to label regions in the mouse and piglet brain with high σ1 receptor density. The specific binding of the enantiomeric tracers [18F](S)-13 and [18F](R)-13 could be replaced by the selective σ1 ligand SA4503.

Characterization of regional binding of clinical amyloid tracers in autosomal dominant and sporadic Alzheimer’s disease brains and their interactions with resveratrol

Characterization of regional binding of clinical amyloid tracers in autosomal dominant and sporadic Alzheimer’s disease brains and their interactions with resveratrol

Can Studies of Neuroinflammation in a TSPO Genetic Subgroup (HAB or MAB) Be Applied to the Entire AD Cohort?

Neuroinflammation plays a significant role in Alzheimer disease (AD), and translocator protein (TSPO) PET imaging allows us to quantify this process. However, the binding of second-generation TSPO tracers depends on the TSPO genotype coded by the rs6971 single-nucleotide polymorphism, with a 40%-50% increase in BP in high-affinity binders (HABs) compared with mixed-affinity binders (MABs), whereas low-affinity binders (LABs) are unsuitable for evaluation. Hence, several studies are using either HAB alone or HAB and MAB subjects. To translate the findings of neuroinflammation studies to the entire population, it is crucial to establish the influence of TSPO genotypes on AD. Here, we investigated whether different TSPO genotypes influence cognitive function, amyloid load, and disease progression over time. We evaluated 798 subjects (225 control, 388 with mild cognitive impairment [MCI], and 185 with AD) from the Alzheimer's Disease Neuroimaging Initiative database at baseline and during follow-up. All subjects were screened for TSPO genotype and underwent detailed clinical and neuropsychologic assessments yearly for 4 y. Of the 798 subjects, 255 also had T1- and T2-weighted MR imaging and amyloid PET with (11)C-Pittsburgh compound B or (18)F-florbetapir. We demonstrated that all TSPO binding groups (HAB, MAB, and LAB) have same level of amyloid load in AD and MCI subjects. We also demonstrated that the prevalence is 50.3% for HAB, 41.2% for MAB, and 8.5% for LAB, without a statistical difference among the AD, MCI, and control groups. During longitudinal follow-up, the mean change in neuropsychometric test scores on the Mini-Mental State Examination, the cognitive and modified Alzheimer Disease Assessment Scales (ADASs), and the Geriatric Depression Scale over time were similar in AD and MCI subjects among the 3 TSPO binding groups. Analysis of the covariates showed that diagnostic group (control, MCI, AD), apolipoprotein E4 status, and sex had a significant effect on decline on the modified Alzheimer Disease Assessment Scale (>3 points of the scale), but age and TSPO genotype did not. This study suggests that information obtained from evaluating a subgroup of AD or MCI subject using second-generation TSPO tracers can be translated to the entire AD and MCI population. Thus, we can study fewer AD subjects in evaluating new antineuroinflammatory and antimicroglial agents in intervention studies and in observational studies evaluating the role of neuroinflammation.

Cortical laminar binding of PET amyloid and tau tracers in Alzheimer disease.

Neurofibrillary tau pathology and amyloid β (Aβ) plaques, characteristic lesions of Alzheimer disease (AD), show different neocortical laminar distributions. Neurofibrillary-tangle tau pathology tends to be closer to the gray matter-white matter boundary, whereas Aβ is dispersed throughout the width of the cortical ribbon. Using PET radiotracers for tau and Aβ lesions, we developed an image analysis tool to measure the distance of tracer-positive voxels from the gray matter-white matter boundary. We studied 5 AD and 5 healthy subjects with both (18)F-THK5117 (tau) and (11)C-Pittsburgh compound B (Aβ) PET. On average, tau-positive voxels were closer to the white matter than were Aβ-positive voxels. This effect was found for all AD subjects and for all regions, both before and after regionally adjusting for the nonspecific white matter binding of both tracers. The differential laminar pattern was validated through postmortem examination. Within cortical lamina, distance measures may be of value in testing PET tracers for their anatomic selectivity.

In vivo binding of the dopamine-1 receptor PET tracers [11C]NNC112 and [11C]SCH23390: a comparison study in individuals with schizophrenia

A deficit in dopamine-1 (D1) receptor function in the prefrontal cortex is suggested to play a role in the cognitive dysfunction observed in patients with schizophrenia. However, the results from positron emission tomography imaging studies of D1 receptor levels in individuals with schizophrenia are mixed. The aim of this investigation was to determine whether the in vivo characteristics of the different D1 receptor tracers used in previous reports, [(11)C]SCH23390 and [(11)C]NNC112, may have contributed to these discrepancies reported in the literature. Eight patients with schizophrenia and 12 healthy control subjects were scanned with both [(11)C]SCH23390 and [(11)C]NNC112. [(11)C]SCH23390 and [(11)C]NNC112 binding potentials in both patients and control subjects were compared and no tracer by diagnosis interactions were observed. The results of this study suggest that differences in the binding of [(11)C]SCH23390 and [(11)C]NNC112 observed in previous studies are not due to differences in the in vivo behavior of these tracers.

Sigma receptor binding of tetrabenazine series tracers targeting VMAT2 in rat pancreas

The vesicular monoamine transporter type II (VMAT2) is highly expressed in pancreatic β-cells and thus has been proposed to be a potential target for measuring β-cell mass (BCM) by molecular imaging. Several tracers based on the TBZ backbone, including 9-fluoropropyl-(+)-dihydrotetrabenazine ([(18)F]AV-133), have shown some promising results as potential biomarkers for BCM despite a relatively high background signal in the pancreas. In the present study, we explore the background binding characteristics of [(18)F]AV-133 in rat pancreas. Pancreatic exocrine cells and islet cells were isolated and purified from Sprague-Dawley rats. Membrane homogenates, prepared from both pancreatic exocrine and islet cells as well as from brain striatum regions, were used for in vitro binding studies of [(18)F]AV-133 under a selective masking condition. 1,3-Di-o-tolylguanidine (DTG), displaying high and roughly equal affinity for both sigma-1 and sigma-2 receptors, was chosen at 5 μM concentration for the masking/blocking studies. [(18)F]AV-133 binding to rat striatum homogenates was not significantly altered by the presence of DTG. In contrast, [(18)F]AV-133 showed significant competition with DTG for binding sites in rat pancreatic exocrine homogenates as well as in rat islet cell homogenates. Importantly, in the presence of DTG, [(18)F]AV-133 showed a single high-affinity binding site on islet cell homogenates with a K(d) value of 3.8 nM which is consistent with the affinity reported previously for VMAT2 sites in rat pancreas. [(18)F]AV-133, in addition to a high-affinity VMAT2 binding site, binds with low affinity (but high capacity) to sigma components that are present in the rat pancreas. Identification of the cause of background binding of [(18)F]AV-133 to rat pancreatic tissue may lead to improved methods for quantification.

Striatal and extrastriatal microPET imaging of D2/D3 dopamine receptors in rat brain with [18F]fallypride and [18F]desmethoxyfallypride

In this study, we compared two different D(2/3) receptor ligands, [¹⁸F]fallypride and [¹⁸F]desmethoxyfallypride ([¹⁸F]DMFP) with respect to the duration of the scan, visualization of extrastriatal receptors, and binding potentials (BP(ND) ) in the rat brain. In addition, we studied the feasibility of using these tracers following a period of awake tracer uptake, during which the animal may perform a behavioral task. Male Sprague-Dawley rats were imaged with [¹⁸F]fallypride and with [¹⁸F]DMFP in four different studies using microPET. All scans were performed under isoflurane anesthesia. The first (test) and second (retest) study were 150-min baseline scans. No retest scans were performed with [¹⁸F]DMFP. A third study was a 60-min awake uptake of radiotracer followed by a 90-min scan. A fourth study was a 150-min competition scan with haloperidol (0.2 mg/kg) administered via tail vein at 90-min post-[¹⁸F]fallypride injection and 60-min post-[¹⁸F]DMFP. For the test-retest studies, BP(ND) was measured using both Logan noninvasive (LNI) method and the interval ratios (ITR) method. Cerebellum was used as a reference region. For the third study, the binding was measured only with the ITR method, and the results were compared to the baseline results. Studies showed that the average transient equilibrium time in the dorsal striatum (DSTR) was at 90 min for [¹⁸F]fallypride and 30 min for [¹⁸F]DMFP. The average BP(ND) for [¹⁸F]fallypride was 14.4 in DSTR, 6.8 in ventral striatum (VSTR), 1.3 in substantia nigra/ventral tegmental area (SN/VTA), 1.4 in colliculi (COL), and 1.5 in central gray area. In the case of [¹⁸F]DMFP, the average BP(ND) values were 2.2 in DSTR, 2.7 in VSTR, and 0.8 in SN/VTA. The haloperidol blockade showed detectable decrease in binding of both tracers in striatal regions with a faster displacement of [¹⁸F]DMFP. No significant changes in BP(ND) of [¹⁸F]fallypride due to the initial awake state of the animal were found, whereas BP(ND) of [¹⁸F]DMFP was significantly higher in the awake state compared to baseline. We were able to demonstrate that dynamic PET using MicroPET Inveon allows quantification of both striatal and extrastriatal [¹⁸F]fallypride binding in rats in vivo. Quantification of the striatal regions could be achieved with [¹⁸F]DMFP.

Further evaluation of quantum chemical methods for the prediction of non-specific binding of positron emission tomography tracers

The non-specific binding of candidate positron emission tomography (PET) radiotracers causes resulting PET images to have poor contrast and is a key determinant for the success or failure of imaging drugs. Non-specific binding is thought to arise when radiotracers bind to cell membranes and moieties other than their intended target. Our previous preliminary work has proposed the use of the drug-lipid interaction energy descriptor to predict the level of non-specific binding in vivo using a limited set of ten well known PET radiotracers with kinetic modelling data taken from the literature. This work validates and extends the use of the drug-lipid interaction energy descriptor using a new set of twenty-two candidate PET radiotracers with non-specific binding data recently collected at the same imaging centre with consistent methodology. As with the previous set of radiotracers, a significant correlation is found between the quantum chemical drug-lipid interaction energy and in vivo non-specific binding experimental values. In an effort to speed up the calculation process, several semi-empirical quantum chemical methods were assessed for their ability to reproduce the ab initio results. However no single semi-empirical method was found to consistently reproduce the level of correlation achieved with ab initio quantum chemical methods.