Kazuhiko Yanai

18F-THK523: a novel in vivo tau imaging ligand for Alzheimer’s disease

While considerable effort has focused on developing positron emission tomography β-amyloid imaging radiotracers for the early diagnosis of Alzheimers disease, no radiotracer is available for the non-invasive quantification of tau. In this study, we detail the characterization of (18)F-THK523 as a novel tau imaging radiotracer. In vitro binding studies demonstrated that (18)F-THK523 binds with higher affinity to a greater number of binding sites on recombinant tau (K18Δ280K) compared with β-amyloid(1-42) fibrils. Autoradiographic and histofluorescence analysis of human hippocampal serial sections with Alzheimers disease exhibited positive THK523 binding that co-localized with immunoreactive tau pathology, but failed to highlight β-amyloid plaques. Micro-positron emission tomography analysis demonstrated significantly higher retention of (18)F-THK523 (48%; P < 0.007) in tau transgenic mice brains compared with their wild-type littermates or APP/PS1 mice. The preclinical examination of THK523 has demonstrated its high affinity and selectivity for tau pathology both in vitro and in vivo, indicating that (18)F-THK523 fulfils ligand criteria for human imaging trials.

2-(2-[2-Dimethylaminothiazol-5-yl]Ethenyl)-6- (2-[Fluoro]Ethoxy)Benzoxazole: A Novel PET Agent for In Vivo Detection of Dense Amyloid Plaques in Alzheimer's Disease Patients

Extensive deposition of dense amyloid fibrils is a characteristic neuropathologic hallmark in Alzheimers disease (AD). Noninvasive detection of these molecules is potentially useful for early and precise detection of patients with AD. This study reports a novel compound, 2-(2-[2-dimethylaminothiazol-5-yl]ethenyl)-6-(2-[fluoro]ethoxy)benzoxazole (BF-227), for in vivo detection of dense amyloid deposits using PET. Methods: The binding affinity of BF-227 to amyloid-β (Aβ) fibrils was calculated. The binding property of BF-227 to amyloid plaques was evaluated by neuropathologic staining of AD brain sections. Brain uptake and in vivo binding of BF-227 to Aβ deposits were also evaluated using mice. For clinical evaluation of 11C-BF-227 as a PET probe, 11 normal (healthy) subjects and 10 patients with AD participated in this study. Dynamic PET images were obtained for 60 min after administration of 11C-BF-227. The regional standardized uptake value (SUV) and the ratio of regional to cerebellar SUV were calculated as an index of 11C-BF-227 retention. The regional tracer distribution in AD patients was statistically compared with that of aged normal subjects on a voxel-by-voxel basis. Results: BF-227 displayed high binding affinity to synthetic Aβ1-42 fibrils (Ki [inhibition constant], 4.3 ± 1.5 nM). Neuropathologic staining has demonstrated preferential binding of this agent to dense amyloid deposits in AD brain. Moreover, a biodistribution study of this agent revealed excellent brain uptake and specific labeling of amyloid deposits in transgenic mice. The present clinical PET study using 11C-BF-227 demonstrated the retention of this tracer in cerebral cortices of AD patients but not in those of normal subjects. All AD patients were clearly distinguishable from normal individuals using the temporal SUV ratio. Voxel-by-voxel analysis of PET images revealed that cortical BF-227 retention in AD patients is distributed primarily to the posterior association area of the brain and corresponded well with the preferred site for neuritic plaque depositions containing dense Aβ fibrils. Conclusion: These findings suggest that BF-227 is a promising PET probe for in vivo detection of dense amyloid deposits in AD patients.

Novel 18F-Labeled Arylquinoline Derivatives for Noninvasive Imaging of Tau Pathology in Alzheimer Disease

Neurofibrillary tangles in Alzheimer disease (AD) brains are composed of the microtubule-associated protein tau. Noninvasive monitoring of tau protein aggregates in the living brain will provide useful information regarding tau pathophysiology in AD. However, no PET probes are currently available for selective detection of tau pathology in AD. We have previously reported 18F-labeled THK-523 (18F-6-(2-fluoroethoxy)-2-(4-aminophenyl)quinoline) as a tau imaging radiotracer candidate for PET. After compound optimization, we developed novel 18F-labeled arylquinoline derivatives, 18F-THK-5105 and 18F-THK-5117, for use as tau imaging PET tracers. Methods: 18F-labeled compounds were prepared from the corresponding tosylated precursors. The binding affinity of compounds to synthetic tau aggregates and tau-rich AD brain homogenates was determined by saturation and competition binding assays. The binding selectivity of compounds to tau pathology was evaluated by autoradiography of AD brain sections. The pharmacokinetics of compounds were assessed in biodistribution studies in normal mice. A 14-d toxicity study with intravenous administration of compounds was performed using rats and mice. Results: In vitro binding assays demonstrated higher binding affinity of THK-5105 and THK-5117 than THK-523 to tau protein aggregates and tau-rich AD brain homogenates. Autoradiographic analyses of AD brain sections showed that these radiotracers preferentially bound to neurofibrillary tangles and neuropil threads, which colocalized with Gallyas-positive and immunoreactive tau protein deposits. The distribution of this radiotracer binding in AD brain sections was completely different from that of 11C-Pittsburgh compound B, showing preferential binding to amyloid plaques. Furthermore, these derivatives demonstrated abundant initial brain uptake and faster clearance in normal mice than 18F-THK-523 and other reported 18F-labeled radiotracers. THK-5105 and THK-5117 showed no toxic effects related to the administration of these compounds in mice and rats and no significant binding for various neuroreceptors, ion channels, and transporters at 1-μM concentrations. Conclusion: 18F-labeled THK-5105 and THK-5117 are promising candidates as PET tau imaging radiotracers.

Quinoline and Benzimidazole Derivatives: Candidate Probes for In Vivo Imaging of Tau Pathology in Alzheimer's Disease

Neurofibrillary tangles (NFTs), neuropil threads, and neuritic elements of senile plaques predominantly comprise hyperphosphorylated tau protein and represent pathological characteristics of Alzheimers disease (AD). These lesions occur before the presentation of clinical symptoms and correlate with the severity of dementia. In vivo detection of these lesions would thus prove useful for preclinical diagnosis of AD and for tracking disease progression. The present study introduces three novel compounds, 4-[2-(2-benzoimidazolyl)ethenyl]-N,N-diethylbenzenamine (BF-126), 2-[(4-methylamino)phenyl]quinoline (BF-158), and 2-(4-aminophenyl)quinoline (BF-170), as candidate probes for in vivo imaging of tau pathology in the AD brain. When solutions of these compounds are injected intravenously into normal mice, these agents exhibit excellent brain uptake and rapid clearance from normal brain tissue. These compounds display relatively lower binding affinity to β-amyloid fibrils and higher binding affinity to tau fibrils, compared with previously reported probe BF-168. In neuropathological examination using AD brain sections, BF-126, BF-158, and BF-170 clearly visualize NFTs, neuropil threads, and paired helical filament-type neuritis. Autoradiography using 11C-labeled BF-158 further demonstrated labeling of NFTs in AD brain sections. These findings suggest the potential usefulness of quinoline and benzimidazole derivatives for in vivo imaging of tau pathology in AD.

Non-invasive assessment of Alzheimer’s disease neurofibrillary pathology using 18F-THK5105 PET

Non-invasive imaging of tau pathology in the living brain would be useful for accurately diagnosing Alzheimers disease, tracking disease progression, and evaluating the treatment efficacy of disease-specific therapeutics. In this study, we evaluated the clinical usefulness of a novel tau-imaging positron emission tomography tracer 18F-THK5105 in 16 human subjects including eight patients with Alzheimers disease (three male and five females, 66-82 years) and eight healthy elderly controls (three male and five females, 63-76 years). All participants underwent neuropsychological examination and 3D magnetic resonance imaging, as well as both 18F-THK5105 and 11C-Pittsburgh compound B positron emission tomography scans. Standard uptake value ratios at 90-100 min and 40-70 min post-injection were calculated for 18F-THK5105 and 11C-Pittsburgh compound B, respectively, using the cerebellar cortex as the reference region. As a result, significantly higher 18F-THK5105 retention was observed in the temporal, parietal, posterior cingulate, frontal and mesial temporal cortices of patients with Alzheimers disease compared with healthy control subjects. In patients with Alzheimers disease, the inferior temporal cortex, which is an area known to contain high densities of neurofibrillary tangles in the Alzheimers disease brain, showed prominent 18F-THK5105 retention. Compared with high frequency (100%) of 18F-THK5105 retention in the temporal cortex of patients with Alzheimers disease, frontal 18F-THK5105 retention was less frequent (37.5%) and was only observed in cases with moderate-to-severe Alzheimers disease. In contrast, 11C-Pittsburgh compound B retention was highest in the posterior cingulate cortex, followed by the ventrolateral prefrontal, anterior cingulate, and superior temporal cortices, and did not correlate with 18F-THK5105 retention in the neocortex. In healthy control subjects, 18F-THK5105 retention was ∼10% higher in the mesial temporal cortex than in the neocortex. Notably, unlike 11C-Pittsburgh compound B, 18F-THK5105 retention was significantly correlated with cognitive parameters, hippocampal and whole brain grey matter volumes, which was consistent with findings from previous post-mortem studies showing significant correlations of neurofibrillary tangle density with dementia severity or neuronal loss. From these results, 18F-THK5105 positron emission tomography is considered to be useful for the non-invasive assessment of tau pathology in the living brain. This technique would be applicable to the longitudinal evaluation of tau deposition and allow a better understanding of the pathophysiology of Alzheimers disease.

Behavioural characterization and amounts of brain monoamines and their metabolites in mice lacking histamine H1 receptors

Behavioural assessments were made of mutant mice lacking histamine H1 receptors to reveal the function of H1 receptors in the behaviour of mice. Exploratory behaviour of mice in a new environment was examined to discover whether the absence of H1 receptors in mice affects actions relating to their emotions. The H1 receptor-deficient mice showed a significant decrease in ambulation in an open field and on an activity wheel. Cognitive functions and anxiety were examined using passive avoidance response test and the elevated plus-maze test, respectively. The passive avoidance test did not show any change in latency. The elevated plus-maze test revealed that the transfer latency of the mutant mice was significantly prolonged, indicating that H1 receptors are partly associated with the control of anxiety. Aggressive behaviour was examined by a resident-intruder aggression test. When confronted with an intruder, the mutant mice attacked the intruder significantly slower and less frequently than did wild-type mice after a six-month isolation period. A formalin test and a forced swimming test were used to evaluate the nociceptive response and depressive or despairing state, respectively, of both groups. The mutant mice showed a significant decrease of nociceptive response in the late phase without affecting the early phase. There was no significant difference in the forced swimming test between the two groups. The brain content of monoamines and their metabolites was measured in the H1 receptor null and wild-type mice. The turnover rate of 5-hydroxytryptamine defined by the ratio of 5-hydroxyindoleacetic acid and 5-hydroxytryptamine was significantly increased in the cerebral cortex and hippocampus of H1 receptor null mice. These results support the previous pharmacological findings that histamine modulates various neurophysiological functions such as locomotor activity, emotion, memory and learning, nociception and aggressive behaviour through H1 receptors.

Imaging of central itch modulation in the human brain using positron emission tomography.

The unpleasantness of itching is reduced by cooling. Although previous research suggests the presence of a central itch modulation system, there is little documentation about the modulation system in the brain. In the present study, we investigated the modulating system of the itching sensation in human brains using positron emission tomography and H2 15O. The significant increases of regional cerebral blood flow caused by histamine stimuli using iontophoresis were observed in the anterior cingulate cortex (BA24), the thalamus, the parietal cortex (BA40 and BA7), the dorsolateral prefrontal cortex (BA46) and the premotor cortex (BA6). We did not observe any changes in the secondary somatosensory cortex (S2) during the itching stimulus, corresponding to the previous imaging studies concerning itching. Activation in these areas related to itching stimuli was decreased by a simultaneous stimulation of itching and cold pain (the dual stimuli), as compared to itching alone. Interestingly, the midbrain, including periaqueductal gray matter (PAG), was only activated during the dual stimuli. PAG is well known to be a modulating noxious stimulus. Here we hypothesize that the activation of PAG may also be related to the itch modulation. These findings indicate that the modified brain activities in the PAG, the cingulate, the frontal and the parietal cortex might be associated with the itch modulation in the central nervous system and that the S2 might not be primarily involved in processing the itching perception in the brain since the activity of S2 was not observed in any concentration of itching stimuli.

Roles of histamine in regulation of arousal and cognition : functional neuroimaging of histamine H1 receptors in human brain

Brain histamine is involved in a wide range of physiological functions such as regulation of the sleep-wake cycle, arousal, cognition, and memory mainly through interactions with histamine H1 receptors (H1Rs). Neurons producing histamine, histaminergic neurons, are exclusively located in the posterior hypothalamus and transmit histamine to almost all regions of the brain. Histamine H1 antagonists, or antihistamines, often prescribed for treatment of allergic disorders, sometimes induce sleepiness and cognitive deficits. It is understood that the mechanism of such CNS side effects is that antihistamine blocks H1Rs in the brain. The purpose of the present study was to compare the CNS side effects of different antihistamines. Subjective sleepiness was measured using the Stanford Sleepiness Scale (SSS) and psychomotor performance was examined by a tachistoscope testing system in healthy, young, Japanese volunteers (16 males, 20-28 yrs.) before and after oral administration of antihistamines such as fexofenadine (FEX) and cetirizine (CET). Additionally, H1R occupancy by antihistamines was examined by PET with 11C-doxepin in 8 volunteers. The results of SSS and psychomotor tests demonstrated that FEX tended to be less sedative than CET though the difference was not statistically significant. PET measurements revealed that no H1Rs in the cerebral cortex were occupied by FEX while about 30% of H1Rs were occupied by CET. In summary, it was confirmed that histamine and H1Rs are involved in maintaining arousal and cognition in humans, and that the severity of clinical symptoms is correlated to the amount of antihistamine that penetrated into the brain.

Consensus Group on New-Generation Antihistamines (CONGA) : present status and recommendations

CONTENTS Prefacey1305 Executive summary and recommendationsy1306

Decreased brain histamine content in hypocretin/orexin receptor-2 mutated narcoleptic dogs.

A growing amount of evidence suggests that a deficiency in hypocretin/orexin neurotransmission is critically involved in animal and human forms of narcolepsy. Since hypocretin-containing neurons innervate and excite histaminergic tuberomammillary neurons, altered histaminergic neurotransmission may also be involved in narcolepsy. We found a significant decrease in histamine content in the cortex and thalamus, two structures important for histamine-mediated cortical arousal, in Hcrtr-2 mutated narcoleptic Dobermans. In contrast, dopamine and norepinephrine contents in these structures were elevated in narcoleptic animals, a finding consistent with our hypothesis of altered catecholaminergic transmission in these animals. Considering the fact that histamine promotes wakefulness, decreases in histaminergic neurotransmission may also account for the sleep abnormalities in hypocretin-deficient narcolepsy.