Positron Emission Tomography Ligand Research Articles

Increased cerebral blood flow after single dose of antipsychotics in healthy volunteers depends on dopamine D2 receptor density profiles

As a result of neuro-vascular coupling, the functional effects of antipsychotics in human brain have been investigated in both healthy and clinical populations using haemodynamic markers such as regional Cerebral Blood Flow (rCBF). However, the relationship between observed haemodynamic effects and the pharmacological action of these drugs has not been fully established. Here, we analysed Arterial Spin Labelling (ASL) rCBF data from a placebo-controlled study in healthy volunteers, who received a single dose of three different D2 receptor (D2R) antagonists and tested the association of the main effects of the drugs on rCBF against normative population maps of D2R protein density and gene-expression data. In particular, we correlated CBF changes after antipsychotic administration with non-displaceable binding potential (BPND) template maps of the high affinity D2-antagonist Positron Emission Tomography (PET) ligand [18F]Fallypride and with brain post-mortem microarray mRNA expression data for the DRD2 gene from the Allen Human Brain Atlas (ABA). For all antipsychotics, rCBF changes were directly proportional to brain D2R densities and DRD2 mRNA expression measures, although PET BPND spatial profiles explained more variance as compared with mRNA profiles (PET R2 range = 0.20–0.60, mRNA PET R2 range 0.04–0.20, pairwise-comparisons all pcorrected<0.05). In addition, the spatial coupling between ΔCBF and D2R profiles varied between the different antipsychotics tested, possibly reflecting differential affinities. Overall, these results indicate that the functional effects of antipsychotics as measured with rCBF are tightly correlated with the distribution of their target receptors in striatal and extra-striatal regions. Our results further demonstrate the link between neurotransmitter targets and haemodynamic changes reinforcing rCBF as a robust in-vivo marker of drug effects. This work is important in bridging the gap between pharmacokinetic and pharmacodynamics of novel and existing compounds.

Development of 2-(2-(3-(4-([18F]Fluoromethoxy- d2)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione for Positron-Emission-Tomography Imaging of Phosphodiesterase 10A in the Brain.

Phosphodiesterase 10A (PDE10A) is a newly identified therapeutic target for central-nervous-system disorders. 2-(2-(3-(4-([18F]Fluoroethoxy)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([18F]MNI-659, [18F]5) is a useful positron-emission-tomography (PET) ligand for imaging of PDE10A in the human brain. However, the radiolabeled metabolite of [18F]5 can accumulate in the brain. In this study, using [18F]5 as a lead compound, we designed four new 18F-labeled ligands ([18F]6-9) to find one more suitable than [18F]5. Of these, 2-(2-(3-(4-([18F]fluoromethoxy- d2)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([18F]9) exhibited high in vitro binding affinity ( Ki = 2.9 nM) to PDE10A and suitable lipophilicity (log D = 2.2). In PET studies, the binding potential (BPND) of [18F]9 (5.8) to PDE10A in the striatum of rat brains was significantly higher than that of [18F]5 (4.6). Furthermore, metabolite analysis showed much lower levels of contamination with radiolabeled metabolites in the brains of rats given [18F]9 than in those given [18F]5. In conclusion, [18F]9 is a useful PET ligand for PDE10A imaging in brain.

Correlation of In Vivo [18F]Flortaucipir With Postmortem Alzheimer Disease Tau Pathology

In Alzheimer disease (AD), tau filaments form neuronal inclusions in neurites (neuropil threads) and in somata (neurofibrillary tangles), and neurite tau pathology constitutes the most common pathology. Positron emission tomography (PET) ligands have been developed to detect in vivo tau pathology in AD. However, the association of AD tau pathology post mortem with in vivo tau PET retention has not been established. Therefore, there is a need to investigate the associations of tau PET with postmortem tau pathology in AD. To study the association of regional in vivo retention of the tau PET ligand [18F]flortaucipir (previously known as AV1451) with the density of tau neuropathology in the corresponding brain regions in a patient with AD. The patient was a man in his 40s with AD caused by a PSEN1 mutation. Between May 2015 and December 2016, he underwent 2 [18F]flortaucipir PET scans at Lund University Hospital, Lund, Sweden. Postmortem analysis was performed 12 months after the last PET scan. Tau pathology was assessed using phosphorylated tau (AT8) immunohistochemistry and Gallyas silver staining. In addition to the regional total tau pathology burden, the density of tau-positive neurites and intrasomal tau tangles were quantified using a stereology-based method. Further, β-amyloid-containing plaques were detected using 4G8 immunohistochemistry. Data were analyzed between January 2018 and August 2018. Regional standardized uptake value ratios of [18F]flortaucipir were compared with the amount of tau pathology in the corresponding brain areas. In this patient, the clinical disease symptoms progressed rapidly in life, paralleled with an annual increase of tau PET retention of 20% to 40% in many cortical regions. Compared with postmortem immunohistochemistry, regional in vivo uptake of [18F]flortaucipir was correlated with the density of tau-positive neurites (AT8: rs = 0.87; P < .001; Gallyas: rs = 0.92; P < .001), intrasomal tau tangles (AT8: rs = 0.65; P = .01; Gallyas: rs = 0.84; P < .001), and total tau burden (AT8: rs = 0.84; P < .001; Gallyas: rs = 0.82; P < .001). No correlations between [18F]flortaucipir and β-amyloid pathology were found. These results indicate that [18F]flortaucipir PET retention is a robust in vivo measure of the total AD tau burden.

Patterns of Progression After 68Ga-PSMA-Ligand PET/CT-Guided Radiation Therapy for Recurrent Prostate Cancer

To determine the patterns of progression after 68Ga prostate-specific membrane antigen (PSMA)-ligand positron emission tomography (PET)/computed tomography (CT)-guided radiation therapy (RT) for recurrent oligometastatic prostate cancer (PCa). One hundred and eight patients with increased prostate-specific antigen levels, who received 68Ga-PSMA-ligand PET/CT-guided RT for recurrent oligometastatic disease after primary therapy for PCa were included. The biochemical progression-free survival and distant disease-free survival after PSMA-ligand PET/CT-guided RT were determined. The patterns of progression were determined using renewed 68Ga-PSMA-ligand PET/CT in patients with biochemical progression and compared with the clinical target volume of the 68Ga-PSMA-ligand PET/CT-guided RT. The frequency of infield and outfield relapses was recorded. A total of 97.2% (105 of 108) of patients showed a decrease in prostate-specific antigen levels after RT and were classified as biochemical responders. After the median follow-up of 18months, 43.5% (47 of 108) of the patients experienced biochemical progression, resulting in an estimated biochemical progression-free survival of 16months. Renewed 68Ga-PSMA-ligand PET/CT allowed localization of recurrent disease in 91.7% (33 of 36) of patients. Analysis of the patterns of progression resulted in a cumulative infield relapse rate of 12.1% (4 of 33) and a cumulative outfield relapse rate of 87.9% (29 of 33). The resultant median disease-free survival was 11months. In terms of the pattern of progression, we observed a shift in the pattern of metastases toward skeletal involvement and distant lymph node metastases. Of these patients, 45.5% (15 of 33) were treated with further RT to delay initiation or escalation of systemic therapies. PSMA-ligand PET/CT-guided RT for relapsed PCa with limited tumor burden allowed individualization of treatment approaches, provided effective local control, and resulted in considerably prolonged biochemical progression-free survival. As indicated by the PSMA-ligand PET/CT-based patterns of progression, repeated PET/CT-guided RT may represent a treatment option in well-selected patients with relapse after RT for oligometastatic disease.

Positron Emission Tomography (PET) Ligand Development for Ionotropic Glutamate Receptors: Challenges and Opportunities for Radiotracer Targeting N-Methyl-d-aspartate (NMDA), α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA), and Kainate Receptors.

Ionotropic glutamate receptors (iGluRs) mediate excitatory neurotransmission within the mammalian central nervous system. iGluRs exist as three main groups: N-methyl-d-aspartate receptors (NMDARs), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and kainate receptors. The past decades have witnessed a remarkable development of PET tracers targeting different iGluRs including NMDARs and AMPARs, and several of the tracers have advanced to clinical imaging studies. Here, we assess the recent development of iGluR PET probes, focusing on tracer design, brain kinetics, and performance in PET imaging studies. Furthermore, this review will not only present challenges in the tracer development but also provide novel approaches in conjunction with most recent drug discovery efforts on these iGluRs, including subtype-selective NMDAR and transmembrane AMPAR regulatory protein modulators and positive allosteric modulators (PAMs) of AMPARs. These approaches, if successful as PET tracers, may provide fundamental knowledge to understand the roles of iGluR receptors under physiological and pathological conditions.

Deuterium Kinetic Isotope Effect Studies of a Potential in Vivo Metabolic Trapping Agent for Monoamine Oxidase B

Visualizing the in vivo activity of monoamine oxidase B (MAO-B) is a valuable tool in the ongoing investigation of astrogliosis in neurodegeneration. Existing strategies for imaging changes in MAO enzyme expression or activity have utilized the irreversible suicide inhibitors or high-affinity reversibly binding inhibitors as positron emission tomography (PET) ligands. As an alternative approach, we developed 4-methyl-7-[(1-[11C]methyl-1,2,3,6-tetrahydropyridin-4-yl)oxy]-2 H-chromen-2-one ([11C]Cou) as a metabolic trapping agent for MAO-B. Trapping of [11C]Cou in rhesus monkey brain demonstrated MAO-B selectivity. In this work, we have attempted to improve on the in vivo pharmacokinetics of [11C]Cou by using the deuterium kinetic isotope effect (KIE) to slow the MAO-B-mediated oxidation step and thus reduce the rate of trapping in brain tissues. However, in vitro assays of enzyme kinetics and in vivo PET imaging of pharmacokinetics in primate brain showed no effects of deuterium substitution on the tetrahydropyridine ring of [11C]Cou. The results are possibly due to masking of the KIE by a second step in the overall metabolism of the new imaging agent.

Characterization of [11C]PXT012253 as a PET Radioligand for mGlu4 Allosteric Modulators in Nonhuman Primates.

Modulation of presynaptic metabotropic glutamate receptor 4 (mGlu4) by an allosteric ligand has been proposed as a promising therapeutic target in Parkinson's disease and levodopa-induced dyskinesia. A positron emission tomography (PET) ligand for an allosteric site of mGlu4 may provide evidence that a clinical drug candidate reaches and binds the target. A carbon-11-labeled PET radioligand binding an allosteric site of mGlu4, [11C]PXT012253, has been recently developed. Here, we describe the detailed characterization of this novel radiolabeled mGlu4 ligand in nonhuman primates. [11C]PXT012253 binding in the brain of cynomolgus monkeys, under the baseline and blocking conditions with the structurally different mGlu4 allosteric ligand PXT002331, currently in clinical trials for Parkinson's disease, was quantified with compartment and graphical modeling approaches using a radiometabolite-corrected plasma input function. Whole-body biodistribution of [11C]PXT012253 was then assessed using PET/x-ray computed tomography to estimate the human effective doses of [11C]PXT012253 for further clinical studies. [11C]PXT012253 displayed binding in mGlu4-expressing regions in the brain of cynomolgus monkeys. Brain regional time-activity curves of [11C]PXT012253 were well described in the two-tissue compartment model (2TC). Total distribution volume was stably estimated using Logan plot and multilinear analysis (MA1) although 2TC showed unstable values in some cases. Competition with PXT002331 showed high specific binding in the total distribution volume. Whole-body PET showed high accumulation of [11C]PXT012253 in the liver, kidney, heart, and brain in the initial phase. The radioligand was excreted through both the gastrointestinal and the urinary tracts. Effective dose of [11C]PXT012253 was estimated to be 0.0042mSv/MBq. [11C]PXT012253 was shown to be a promising PET radioligand for mGlu4 allosteric modulators in the monkey brain. MA1 would be the choice of quantitative method. Further development of [11C]PXT012253 in human subjects is warranted.

First in human evaluation of [18F]PK-209, a PET ligand for the ion channel binding site of NMDA receptors

BackgroundEfforts to develop suitable positron emission tomography (PET) tracers for the ion channel site of human N-methyl-d-aspartate (NMDA) receptors have had limited success. [18F]PK-209 is a GMOM derivative that binds to the intrachannel phencyclidine site with high affinity and selectivity. Primate PET studies have shown that the volume of distribution in the brain was reduced by administration of the NMDA receptor antagonist MK-801, consistent with substantial specific binding. The purpose of the present study was to evaluate [18F]PK-209 in 10 healthy humans by assessing test–retest reproducibility and binding specificity following intravenous S-ketamine administration (0.5 mg ∙ kg−1). Five healthy subjects underwent a test–retest protocol, and five others a baseline-ketamine protocol. In all cases dynamic, 120-min PET scans were acquired together with metabolite-corrected arterial plasma input functions. Additional input functions were tested based on within-subject and population-average parent fractions.ResultsBest fits of the brain time-activity curves were obtained using an irreversible two-tissue compartment model with additional blood volume parameter. Mean test–retest variability of the net rate of influx Ki varied between 7 and 24% depending on the input function. There were no consistent changes in [18F]PK-209 PET parameters following ketamine administration, which may be a consequence of the complex endogenous ligand processes that affect channel gating.ConclusionsThe molecular interaction between [18F]PK-209 and the binding site within the NMDA receptor ion channel is insufficiently reproducible and specific to be a reliable imaging agent for its quantification.Trial registrationEudraCT 2014-001735-36. Registered 28 April 2014

Use of the PET ligand florbetapir for in vivo imaging of pancreatic islet amyloid deposits in hIAPP transgenic mice.

Islet amyloid deposits contribute to beta cell dysfunction and death in most individuals with type 2 diabetes but non-invasive methods to determine the presence of these pathological protein aggregates are currently not available. Therefore, we examined whether florbetapir, a radiopharmaceutical agent used for detection of amyloid-β deposits in the brain, also allows identification of islet amyloid in the pancreas. Saturation binding assays were used to determine the affinity of florbetapir for human islet amyloid polypeptide (hIAPP) aggregates in vitro. Islet amyloid-prone transgenic mice that express hIAPP in their beta cells and amyloid-free non-transgenic control mice were used to examine the ability of florbetapir to detect islet amyloid deposits in vitro, in vivo and ex vivo. Mice or mouse pancreases were subjected to autoradiographic, histochemical and/or positron emission tomography (PET) analyses to assess the utility of florbetapir in identifying islet amyloid. In vitro, florbetapir bound synthetic hIAPP fibrils with a dissociation constant of 7.9nmol/l. Additionally, florbetapir bound preferentially to amyloid-containing hIAPP transgenic vs amyloid-free non-transgenic mouse pancreas sections in vitro, as determined by autoradiography (16,475 ± 5581 vs 5762 ± 575 density/unit area, p < 0.05). In hIAPP transgenic and non-transgenic mice fed a high-fat diet for 1year, intravenous administration of florbetapir followed by PET scanning showed that the florbetapir signal was significantly higher in amyloid-laden hIAPP transgenic vs amyloid-free non-transgenic pancreases in vivo during the first 5min of the scan (36.83 ± 2.22 vs 29.34 ± 2.03 standardised uptake value × min, p < 0.05). Following PET, pancreases were excised and florbetapir uptake was determined ex vivo by γ counting. Pancreatic uptake of florbetapir was significantly correlated with the degree of islet amyloid deposition, the latter assessed by histochemistry (r = 0.74, p < 0.001). Florbetapir binds to islet amyloid deposits in a specific and quantitative manner. In the future, florbetapir may be useful as a non-invasive tool to identify islet amyloid deposits in humans.

PET Imaging of T Cells: Target Identification and Feasibility Assessment

Imaging T cells using positron emission tomography (PET) would be highly useful for diagnosis and monitoring in immunology and oncology patients. There are, however, no obvious targets that can be used to develop imaging agents for this purpose. We evaluated several potential target proteins with selective expression in T cells, and for which lead molecules were available: protein kinase C isozyme θ (PKC θ), lymphocyte-specific protein tyrosine kinase (Lck), zeta-chain-associated protein kinase 70 (ZAP70), and interleukin-2-inducible T-cell kinase (Itk). Ultimately, we focused on Itk and identified a tool molecule with properties suitable for in vivo imaging of T cells: (5aR)-5,5-difluoro-5a-methyl-N-(1-((S)-3-(methylsulfonyl)phenyl)(tetrahydro-2H-pyran-4-yl)methyl)-1H-pyrazol-4-yl)-1,4,4a,5,5a,6-hexahydrocyclopropa[f]indazole-3-carboxamide (23). Although it does not have the optimal profile for clinical use, this molecule indicates that it might be possible to develop Itk-selective PET ligands for imaging the distribution of T cells in patients.

Update on biomarkers for amyloid pathology in Alzheimer's disease.

At the center of Alzheimer's disease pathogenesis is the aberrant aggregation of amyloid-β (Aβ) into oligomers, fibrils and plaques. Effective monitoring of Aβ deposition directly in patients is essential to assist anti-Aβ therapeutics in target engagement and participant selection. In the advent of approved anti-Aβ therapeutics, biomarkers will become of fundamental importance in initiating treatments having disease modifying effects at the earliest stage. Two well-established Aβ biomarkers are widely utilized: Aβ-binding ligands for positron emission tomography and immunoassays to measure Aβ42 in cerebrospinal fluid. In this review, we will discuss the current clinical, diagnostic and research state of biomarkers for Aβ pathology. Furthermore, we will explore the current application of blood-based markers to assess Aβ pathology.

A Systematic Review of Positron Emission Tomography of Tau, Amyloid Beta, and Neuroinflammation in Chronic Traumatic Encephalopathy: The Evidence To Date.

Chronic traumatic encephalopathy (CTE) is associated with pathological changes, yet detecting these changes during life has proven elusive. Positron emission tomography (PET) offers the potential for identifying such pathology. Few studies have been completed to date and their approaches and results have been diverse. It was the objective of this review to systematically examine relevant research using ligands for PET that bind to identified pathology in CTE. We focused on identification of patterns of binding and addressing gaps in knowledge of PET imaging for CTE. A comprehensive literature search was conducted. Data used were published on or before May 22, 2017. As the extant literature is limited, any peer-reviewed article assessing military, contact sports athletes, or professional fighters was considered for inclusion. The main outcomes were regional binding to brain regions identified through control comparisons or through clinical metrics (e.g., standardized uptake volume ratios). A total of 1207 papers were identified for review, of which six met inclusion criteria. Meta-analyses were planned but were deemed inappropriate given the small number of studies identified. Methodological concerns in these initial papers included small sample sizes, lack of a control comparison, use of nonstandard statistical procedures to quantify data, and interpretation of potentially off-target binding areas. Across studies, the hippocampi, amygdalae, and midbrain had reasonably consistent increased uptake. Evidence for increased uptake in cortical regions was less consistent. The evidence suggests that the field of PET imaging in those at risk for CTE remains nascent. As the field evolves to include more stringent studies, ligands for PET may prove an important tool in identifying CTE in vivo.

Development of Positron Emission Tomography Radiotracers for the GABA Transporter 1.

In vivo positron emission tomography (PET) imaging of the γ-aminobutyric acid (GABA) receptor complex has been accomplished using radiolabeled benzodiazepine derivatives, but development of specific presynaptic radioligands targeting the neuronal membrane GABA transporter type 1 (GAT-1) has been less successful. The availability of new structure-activity studies of GAT-1 inhibitors and the introduction of a GAT-1 inhibitor (tiagabine, Gabatril) into clinical use prompted us to reinvestigate the syntheses of PET ligands for this transporter. Initial synthesis and rodent PET studies of N-[11C]methylnipecotic acid confirmed the low brain uptake of that small and polar molecule. The common design approach to improve blood-brain barrier permeability of GAT-1 inhibitors is the attachment of a large lipophilic substituent. We selected an unsymmetrical bis-aromatic residue attached to the ring nitrogen by a vinyl ether spacer from a series recently reported by Wanner and coworkers. Nucleophilic aromatic substitution of an aryl chloride precursor with [18F]fluoride was used to prepare the desired candidate radiotracer ( R, E/ Z)-1-(2-((4-fluoro-2-(4-[18F]fluorobenzoyl)styryl)oxy)ethyl)piperidine-3-carboxylic acid (( R, E/ Z)-[18F]10). PET studies in rats showed no brain uptake, which was not altered by pretreatment of animals with the P-glycoprotein inhibitor cyclosporine A, indicating efflux by Pgp was not responsible. Subsequent PET imaging studies of ( R, E/ Z)-[18F]10 in rhesus monkey brain showed very low brain uptake. Finally, to test if the free carboxylic acid group was the likely cause of poor brain uptake, PET studies were done using the ethyl ester derivative of ( R, E/ Z)-[18F]10. Rapid and significant monkey brain uptake of the ester was observed, followed by a slow washout over 90 min. The blood-brain barrier permeability of the ester supports a hypothesis that the free acid function limits brain uptake of nipecotic acid-based GAT-1 radioligands, and future radiotracer efforts should investigate the use of carboxylic acid bioisosteres.

Serotonin, β-amyloid, and cognition in Parkinson disease.

Serotoninergic neurotransmission may modulate β-amyloid peptide (Aβ) metabolism through upregulation of α-secretase. Early Parkinson disease (PD) shows variable serotoninergic denervation, which may impact Aβ deposition. We conducted 3 analyses to explore associations between serotoninergic neurotransmission and cerebral Aβ burden in PD. The first was a cross-sectional imaging study of PD subjects (n = 23) using the serotoninergic transporter positron emission tomography (PET) ligand [11 C]3-amino-4-(2-dimethylaminomethyl-phenylsulfaryl)-benzonitrile (DASB) and amyloid PET Pittsburgh compound B ([11 C]PiB). The second was a baseline study of Parkinson's Progression Markers Initiative (PPMI) subjects exploring the influence of serotoninergic medications on cerebrospinal fluid (CSF) Aβ-42 levels (n = 389), controlling for age, sex, Geriatric Depression Scale, disease duration, and education. Third, we fit an interval censored proportional hazard model with longitudinal PPMI data (n = 367) to test whether serotoninergic medication use associates with reduced risk of PD cognitive decline, defined as time to reach a Montreal Cognitive Assessment score ≤ 20, adjusting for baseline caudate dopamine transporter [123 I]ioflupane single photon emission computed tomography and CSF Aβ-42 levels. Serotoninergic DASB distribution volume ratio (DVR) inversely associated with PiB DVR in the cerebral cortex (Pearson r = -0.478, p = 0.021) but not the striatum (r = -0.264, p = 0.224). In the baseline PPMI analysis, serotoninergic medication use for ≥6 months associated with a lower level of CSF Aβ-42 (t = -2.20, p = 0.029). In the longitudinal PPMI model, baseline serotoninergic medication use associated with a reduced risk of cognitive decline (t = -2.03, p = 0.043) after controlling for covariates. Cortical Aβ burden in PD associates inversely with serotoninergic innervation. Serotoninergic medications may alter Aβ metabolism and reduce the risk of PD cognitive decline. Ann Neurol 2018;83:994-1002.

Large inter- and intra-case variability of first generation tau PET ligand binding in neurodegenerative dementias

Imaging of pathological tau with positron emission tomography (PET) has the potential to allow early diagnosis of the dementias and monitoring of disease progression, including assessment of therapeutic interventions, in vivo. The first generation of tau PET tracers, including the carbazole flortaucipir and the 2-arylquinolines of the THK series, are now used in clinical research; however, concerns have been raised about off-target binding and low sensitivity.With the aim to determine the nature of tau pathology depicted by structurally distinct tau ligands we carried out a microscopic neuropathological evaluation in post-mortem human brain tissue of cases with primary and secondary tauopathies. Carbazole and 2-arylquinoline binding was only observed in cases with Alzheimer’s disease and one case with frontotemporal dementia and parkinsonism linked to chromosome 17 exhibiting a R406W MAPT mutation. In end stage Alzheimer’s disease cases, fluorescent imaging with the carbazole T726 and the 2-arylquinoline THK-5117 revealed high inter- and intra-case variability of tracer binding, and this was corroborated by quantitative phosphorimaging with the PET tracer [18F]THK-5117. Microscopic analysis of the pathological inclusions revealed that the fluorescent tracers preferentially bind to premature tau aggregates. Whilst T726 binding was limited to neuronal tau, THK-5117 additionally depicted neuritic tau. Neither tracer depicted tau in pre-symptomatic disease.Our results highlight limitations of the first generation of tau PET tracers, in particular lack of correlation between pathological tau load and tracer binding, limited sensitivity to tau in early disease, and high variability in tracer binding between and within cases. Concerns remain that these limitations may also affect the next generation tracers as they target the same high affinity binding site. Therefore, it is crucial to assess inter- and intra-subject correlation of tracer binding with pathological tau load in post-mortem tissue studies, and to rigorously assess novel tau PET tracers before translation into clinical studies.

Regional Distribution, Asymmetry, and Clinical Correlates of Tau Uptake on [18F]AV-1451 PET in Atypical Alzheimer's Disease.

Despite common pathology, Alzheimer's disease (AD) can have multiple clinical presentations which pathological studies suggest result from differences in the regional distribution of tau pathology. Positron emission tomography (PET) ligands are now available that can detect tau proteins in vivo and hence can be used to investigate the biological mechanisms underlying atypical AD. To assess regional patterns of tau uptake on PET imaging in two atypical AD variants, posterior cortical atrophy (PCA) and logopenic progressive aphasia (lvPPA). Eighteen PCA and 19 lvPPA subjects that showed amyloid-β deposition on PET underwent tau-PET imaging with [18F]AV-1451. Group comparisons of tau uptake in PCA and lvPPA were performed using voxel-level and regional-level analyses. We also assessed the degree of lobar tau asymmetry and correlated regional tau uptake to age and performance on clinical evaluations. Both syndromes showed diffuse tau uptake throughout all cortical regions, although PCA showed greater uptake in occipital regions compared to lvPPA, and lvPPA showed greater uptake in left frontal and temporal regions compared to PCA. While lvPPA showed predominant left-asymmetric tau deposition, PCA was more bilateral. Younger subjects showed greater tau uptake bilaterally in frontal and parietal lobes than older subjects, and sentence repetition, Boston naming test, simultanagnosia, and visuoperceptual function showed specific regional tau correlates. Tau deposition is closely related to clinical presentation in atypical AD with age playing a role in determining the degree of cortical tau deposition.

Tau Imaging in Parkinsonism: What Have We Learned So Far?

Positron emission tomography ligands are now available that bind to tau proteins in the brain, providing the exciting opportunity to assess the presence and distribution of tau in vivo in living patients. This manuscript performed a systematic review of studies that have performed tau PET imaging in patients with parkinsonian disorders. Pubmed was searched up to November 2017, and the review included case reports and patient-control studies. Most tau-PET studies have utilized the [18F]AV-1451 ligand, with a few using the [11C]PBB3 and [18F]THK-5351 ligands. Elevated cortical tau-PET uptake has been observed in Parkinson's disease dementia and dementia with Lewy bodies, presumed to be related to Alzheimer's disease-related pathology. Mild patterns of tau-PET uptake have been observed in subcortical structures in progressive supranuclear palsy and subcortical structures and motor cortex in corticobasal syndrome, although discrepancy with autoradiographic studies that show lack of binding to 4-repeat tau and "off-target" binding observed in subcortical structures limits interpretation of these findings. Findings in frontotemporal dementia with tau mutations are variable, but elevated signal is most pronounced in mutations with deposition of both 3 and 4-repeat tau. Elevated tau-PET uptake has also been observed in multiple system atrophy, a synucleinopathy. The value of the current generation of tau-PET ligands varies across Parkinsonian syndromes depending upon underlying variability in tau pathology and "off-target" binding. More work is needed to understand the biological basis of binding and more specific tau PET ligands are needed to study parkinsonian disorders.

Tau Filaments and the Development of Positron Emission Tomography Tracers.

A pathological pathway leading from soluble, monomeric to insoluble, filamentous Tau, is believed to underlie human Tauopathies. Cases of frontotemporal dementia are caused by dominantly inherited mutations in MAPT, the Tau gene. They show that dysfunction of Tau protein is sufficient to cause neurodegeneration and dementia. Extrapolation to the more common sporadic Tauopathies leads one to conclude that the pathological pathway is central to the development of all cases of disease, even if there are multiple reasons for Tau assembly. These findings are conceptually similar to those reported for beta-amyloid, alpha-synuclein and prion protein. Here, we provide an overview of Tau filaments and their positron emission tomography ligands.

Advances in the Development of PET Ligands Targeting Histone Deacetylases for the Assessment of Neurodegenerative Diseases.

Epigenetic alterations of gene expression have emerged as a key factor in several neurodegenerative diseases. In particular, inhibitors targeting histone deacetylases (HDACs), which are enzymes responsible for deacetylation of histones and other proteins, show therapeutic effects in animal neurodegenerative disease models. However, the details of the interaction between changes in HDAC levels in the brain and disease progression remain unknown. In this review, we focus on recent advances in development of radioligands for HDAC imaging in the brain with positron emission tomography (PET). We summarize the results of radiosynthesis and biological evaluation of the HDAC ligands to identify their successful results and challenges. Since 2006, several small molecules that are radiolabeled with a radioisotope such as carbon-11 or fluorine-18 have been developed and evaluated using various assays including in vitro HDAC binding assays and PET imaging in rodents and non-human primates. Although most compounds do not readily cross the blood-brain barrier, adamantane-conjugated radioligands tend to show good brain uptake. Until now, only one HDAC radioligand has been tested clinically in a brain PET study. Further PET imaging studies to clarify age-related and disease-related changes in HDACs in disease models and humans will increase our understanding of the roles of HDACs in neurodegenerative diseases.

Efficacy, Predictive Factors, and Prediction Nomograms for 68Ga-labeled Prostate-specific Membrane Antigen–ligand Positron-emission Tomography/Computed Tomography in Early Biochemical Recurrent Prostate Cancer After Radical Prostatectomy

Recently, 68Ga-labeled prostate-specific membrane antigen (PSMA)–ligand positron-emission tomography (PET) imaging has been shown to improve detection rates in recurrent prostate cancer (PC). However, published studies include only small patient numbers at low prostate-specific antigen (PSA) values. For this study, 272 consecutive patients with biochemical recurrence after radical prostatectomy and PSA value between 0.2 and 1ng/ml were included. The 68Ga-PSMA-ligand PET/computed tomography (CT) was evaluated, and detection rates were determined and correlated to various clinical variables using univariate and multivariable analyses. Subgroups of patients with very low (0.2–0.5ng/ml) and low (>0.5–1.0ng/ml) PSA values were analyzed. In total, lesions indicative of PC recurrence were detected in 55% (74/134) and 74% (102/138) with very low and low PSA values, respectively. Main sites of recurrence were pelvic or retroperitoneal lymph nodes metastases, followed by local recurrence and bone metastases with higher probability in the low versus very low PSA subgroup. Detection rates significantly increased with higher PSA values, primary pT≥3a, primary pN+ disease, grade group ≥4, previous radiation therapy, and concurrent androgen deprivation therapy (ADT) in univariate analysis. In a multivariable logistic regression model, concurrent ADT and PSA values were identified as most relevant predictors of positive 68Ga-PSMA-ligand PET/CT. Further, prediction nomograms were established, which may help in estimating pretest PSMA-ligand PET positivity in clinical practice. Patient summaryIn our study, 68Ga-labeled prostate-specific membrane antigen (PSMA)–ligand positron-emission tomography (PET)/computed tomography (CT) detected recurrent disease after radical prostatectomy in 55% (74/134) and 74% (102/138) of patients with very low (0.2–0.5ng/ml) and low (>0.5–1.0ng/ml) prostate-specific antigen values, respectively. On the basis of these data, it seems reasonable to perform 68Ga-PSMA-ligand PET/CT also in patients with early biochemical recurrence, as it can tailor further therapy decisions (eg, local vs systemic treatment). The established prediction nomograms can further assist urologists in discussions on the use of 68Ga-PSMA-ligand PET/CT with their patients in specific clinical settings.