Kazutoshi Suzuki

Linearized Reference Tissue Parametric Imaging Methods: Application to [11C]DASB Positron Emission Tomography Studies of the Serotonin Transporter in Human Brain:

The authors developed and applied two new linearized reference tissue models for parametric images of binding potential (BP) and relative delivery (R1) for [11C]DASB positron emission tomography imaging of serotonin transporters in human brain. The original multilinear reference tissue model (MRTMO) was modified (MRTM) and used to estimate a clearance rate (k′2) from the cerebellum (reference). Then, the number of parameters was reduced from three (MRTM) to two (MRTM2) by fixing k′2. The resulting BP and R1 estimates were compared with the corresponding nonlinear reference tissue models, SRTM and SRTM2, and one-tissue kinetic analysis (1TKA), for simulated and actual [11C]DASB data. MRTM gave k′2 estimates with little bias (<1%) and small variability (<6%). MRTM2 was effectively identical to SRTM2 and 1TKA, reducing BP bias markedly over MRTMO from 12–70% to 1–4% at the expense of somewhat increased variability. MRTM2 substantially reduced BP variability by a factor of two or three over MRTM or SRTM. MRTM2, SRTM2, and 1TKA had R1 bias <0.3% and variability at least a factor of two lower than MRTM or SRTM. MRTM2 allowed rapid generation of parametric images with the noise reductions consistent with the simulations. Rapid parametric imaging by MRTM2 should be a useful method for human [11C]DASB positron emission tomography studies.

Measurement of acetylcholinesterase by positron emission tomography in the brains of healthy controls and patients with Alzheimer's disease

BACKGROUND Acetylcholinesterase activity, a marker for degeneration of the central cholinergic system, has consistently been reported, in necropsy brain studies, to be reduced in the cerebral cortex of patients with Alzheimers disease. We have shown regional acetylcholinesterase activity in vivo in rodent and primate brains with radioactive acetylcholine analogues. In the present study, we used one of the analogues to map acetylcholinesterase activity in the brains of living people. METHODS Positron emission tomography (PET) and a radiolabelled acetylcholine analogue with high hydrolytic specificity to acetylcholinesterase [11C]N-methyl-4-piperidyl acetate (MP4A), was used in eight elderly healthy controls and five patients with Alzheimers disease who had mild dementia. All participants were given an intravenous injection of [11C]MP4A and then sequential patterns of radioactivity in various brain regions were obtained by PET. Time courses of [11C]MP4A concentration in arterial blood were also measured to obtain an input function. A three-compartment model was used to estimate regional acetylcholinesterase activity in the brain. FINDINGS The estimated acetylcholinesterase distribution in the brain of the control participants agreed with the acetylcholinesterase distribution at necropsy. All patients with Alzheimers disease had multiple cortical regions with a reduced estimated acetylcholinesterase activity in comparison with control participants. The reduction was more pronounced in the parietotemporal cortex, with an average reduction rate of 31% in temporal and 38% in parietal cortex, and less pronounced in other cortical lesions (19% in frontal, 24% in occipital, and 20% in sensorimotor cortex). Each patient was found to have at least two cortical regions with significantly reduced acetylcholinesterase activity. INTERPRETATION The method we describe for non-invasive in-vivo detection of regional acetylcholinesterase changes in the living human brain that is feasible for biochemical assessment of Alzheimers disease.

Longitudinal, Quantitative Assessment of Amyloid, Neuroinflammation, and Anti-Amyloid Treatment in a Living Mouse Model of Alzheimer's Disease Enabled by Positron Emission Tomography

We provide the first evidence for the capability of a high-resolution positron emission tomographic (PET) imaging system in quantitatively mapping amyloid accumulation in living amyloid precursor protein transgenic (Tg) mice. After the intravenous administration of N-[11C]methyl-2-(4′-methylaminophenyl)-6-hydroxybenzothiazole (or [11C]PIB for “Pittsburgh Compound-B”) with high-specific radioactivity, the Tg mice exhibited high-level retention of radioactivity in amyloid-rich regions. PET investigation for Tg mice over an extended range of ages, including longitudinal assessments, demonstrated age-dependent increase in radioligand binding consistent with progressive amyloid accumulation. Reduction in amyloid levels in the hippocampus of Tg mice was also successfully monitored by multiple PET scans along the time course of anti-amyloid treatment using an antibody against amyloid β peptide (Aβ). Moreover, PET scans with [18F]fluoroethyl-DAA1106, a radiotracer for activated glia, were conducted for these individuals parallel to amyloid imaging, revealing treatment-induced neuroinflammatory responses, the magnitude of which intimately correlated with the levels of pre-existing amyloid estimated by [11C]PIB. It is also noteworthy that the localization and abundance of [11C]PIB autoradiographic signals were closely associated with those of N-terminally truncated and modified Aβ, AβN3-pyroglutamate, in Alzheimers disease (AD) and Tg mouse brains, implying that the detectability of amyloid by [11C]PIB positron emission tomography is dependent on the accumulation of specific Aβ subtypes. Our results support the usefulness of the small animal-dedicated PET system in conjunction with high-specific radioactivity probes and appropriate Tg models not only for clarifying the mechanistic properties of amyloidogenesis in mouse models but also for preclinical tests of emerging diagnostic and therapeutic approaches to AD.

AGE-RELATED CHANGES IN HUMAN D1 DOPAMINE RECEPTORS MEASURED BY POSITRON EMISSION TOMOGRAPHY

The effects of age on the binding parameters of11C-SCH23390, the highly selective ligand for central D1 dopamine receptors, at specific binding sites in the brain were studied. Seventeen healthy male volunteers (20–72 years old) participated. Regional radioactivity in the brain was followed for 40 min by positron emission tomography (PET). A high accumulation of radioactivity was observed in the striatum and there was a conspicuous accumulation in the neocortex. A two-compartment model was used to obtain quantitative estimates of rate constants of association (k3) and dissociation (k4). The binding potential (k3/k4) of the dopamine D1 receptors in the striatum and frontal cortex decreased by 35% and 39%, respectively, with age. The value of k3 decreased by 58% in the striatum and 83% in the frontal cortex, whereas the value of k4 decreased by 35% in the striatum and 72% in the frontal cortex with age.

Dopamine D2 receptors in the insular cortex and the personality trait of novelty seeking.

Human personality has been considered to have a neurochemical background. We examined the relation between extrastriatal dopamine D2 receptor binding in living human brain and the personality trait of novelty seeking that has been proposed to be related to dopaminergic function in the brain. We measured extrastriatal dopamine D2 receptors of 24 healthy young male subjects using [(11)C]FLB 457 positron emission tomography. The personality trait of each subject was assessed by the Temperament and Character Inventory (TCI). Correlation of dopamine D2 receptor binding with novelty seeking was calculated using region-of-interest analysis and statistical parametric mapping based on the binding potential images generated using a reference tissue model. A significant negative correlation was observed between binding potential values and the novelty seeking scores on TCI in the right insular cortex. No significant correlation was observed in any other region. Our result indicates that there is a significant association between dopamine D2 receptor binding and the human novelty seeking trait in the right insular cortex.

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

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

Imaging of Peripheral Benzodiazepine Receptor Expression as Biomarkers of Detrimental versus Beneficial Glial Responses in Mouse Models of Alzheimer's and Other CNS Pathologies

We demonstrate the significance of peripheral benzodiazepine receptor (PBR) imaging in living mouse models of Alzheimers disease (AD) as biomarkers and functional signatures of glial activation. By radiochemically and immunohistochemically analyzing murine models of the two pathological hallmarks of AD, we found that AD-like Aβ deposition is concurrent with astrocyte-dominant PBR expression, in striking contrast with nonastroglial PBR upregulation in accumulations of AD-like phosphorylated tau. Because tau-induced massive neuronal loss was distinct from the marginal neurodegeneration associated with Aβ plaques in these models, cellular localization of PBR reflected deleterious and beneficial glial reactions to tau versus Aβ pathologies, respectively. This notion was subsequently examined in models of various non-AD neuropathologies, revealing the following reactive glial dynamics underlying differential PBR upregulation: (1) PBR(−) astrogliosis uncoupled with microgliosis or coupled with PBR(+) microgliosis associated with irreversible neuronal insults; and (2) PBR(+) astrogliosis coupled with PBR(− or ±) microgliosis associated with minimal or reversible neuronal toxicity. Intracranial transplantation of microglia also indicated that nontoxic microglia drives astroglial PBR expression. Moreover, levels of glial cell line-derived neurotrophic factor (GDNF) in astrocytes were correlated with astroglial PBR, except for increased GDNF in PBR(-) astrocytes in the model of AD-like tau pathology, thereby suggesting that PBR upregulation in astrocytes is an indicator of neurotrophic support. Together, PBR expressions in astrocytes and microglia reflect beneficial and deleterious glial reactions, respectively, in diverse neurodegenerative disorders including AD, pointing to new applications of PBR imaging for monitoring the impact of gliosis on the pathogenesis and treatment of AD.

Ketamine increases the striatal N-[11C]methylspiperone binding in vivo: positron emission tomography study using conscious rhesus monkey

A system for positron emission tomography study of conscious monkeys was newly developed. By use of this system in combination with a microdialysis technique, the effect of ketamine on the binding and release of dopamine was investigated. The administration of ketamine (5 mg/kg) caused sedation accompanied by psychotic symptoms such as nystagmus and stereotyped movements of extremities. During this psychotomimetic period produced by ketamine, a significant increase in the accumulation of the dopamine D2 receptor ligand N-[11C]methylspiperone was observed in the striatum compared with the level in the conscious state, while no significant change was observed in the frontal cortex and cerebellum. In contrast to the use of ketamine as the anesthetic, pentobarbital (25 mg/kg), which produced deeper anesthesia but no psychotic symptoms, caused a decrease in the accumulation of N-[11C]methylspiperone in the striatum. Kinetic analysis, conducted by a graphical method, revealed that the value of the association constant (K3) for N-[11C]methylspiperone binding in the striatum was increased to approximately 130% by ketamine and decreased to approximately 70% by pentobarbital compared with the control values. Furthermore, the release of dopamine from the striatum measured by microdialysis was not affected by ketamine anesthesia. These results indicate that ketamine facilitates striatal dopaminergic neurotransmission through increasing the binding activity of dopamine D2 receptors in the striatum, and suggest that these changes may be related to the psychotomimetic behavioral symptoms of this drug.

Phase-dependent roles of reactive microglia and astrocytes in nervous system injury as delineated by imaging of peripheral benzodiazepine receptor.

Elevated levels of peripheral benzodiazepine receptor (PBR) in glia have been documented in diverse nervous system injuries, while the identity and spatiotemporal characteristics of the cells showing upregulation of PBR remain elusive. We examined the astrocytic and microglial expressions of PBR in rat brains during the duration of ethanol-induced neuronal insults in order to clarify the significance of PBR as a biomarker capable of detecting a distinctive subpopulation of these glial cells involved in the impairment and protection of neurons. The levels of PBR, as determined by autoradiographic analysis using a specific radioligand, [11C]DAA1106, began to significantly increase at 3 days after intrastriatal injection of ethanol, and peaked at 7 days. This was consistent with the results of double immunofluorescence staining and high-resolution emulsion autoradiography, which revealed upregulation of PBR in both microglia and astrocytes proliferating in nonoverlapping compartments of the injury site. Notably, increased expression of PBR in astrocytes was transiently observed in a manner parallel to the centripetal migration of these cells to the inflammatory lesion, which may be a response indispensable to the protection of intact tissue. Thereafter, astrocytic PBR was barely detectable, despite the presence of numerous glial fibrillary acidic protein-immunoreactive astrocytes forming glial scarring. By contrast, intense PBR signals were persistently present in microglia localized to the injury epicenter up to 90 days, notwithstanding a gradual reduction in the number of ionized calcium binding adapter molecule-1-positive amoeboid microglia between 7 and 90 days. The long-lasting PBR expression in microglia was finally supported by in vivo positron emission tomography imaging, and suggests that inflammatory tissue damage is potentially expandable unless it is tightly sealed by astrocytic scar. The present findings collectively support the utility of PBR in identifying a unique temporal pattern of astrocytic and microglial activation that conventional glial markers hardly pursue.

Computer-controlled large scale production of high specific activity [11C]RO 15-1788 for PET studies of benzodiazepine receptors.

Ethyl 8-fluoro-5,6-dihydro-5-[11C]methyl-6-oxo-4H-imidazo [1,5-a] [1,4]benzodiazepine-3-carboxylate ([11C]RO 15-1788) has been prepared automatically with high specific activity for in vivo visualization or quantitative analysis of brain benzodiazepine receptors. The yield, radiochemical yield, radiochemical purity and specific activity of the product ready for an i.v. injection were 276 +/- 76 mCi, 50.8 +/- 7.8%, 99.3 +/- 0.3% and 2.9 +/- 0.5 Ci/mumol, respectively, taking an average of the latest 3 runs. The time required was about 25 min. Each product was sufficient to carry out three successive clinical studies by positron emission tomography (PET). All the procedures other than evaporation and filtration at the final stage were carried out with specially designed equipment connected to a central control system for radioisotope production.