Chie Seki

PET Quantification of Tau Pathology in Human Brain with 11C-PBB3

Tau accumulation in the brain is a pathologic hallmark of Alzheimer disease and other tauopathies. Quantitative visualization of tau pathology in humans can be a powerful method as a diagnostic aid and for monitoring potential therapeutic interventions. We established methods of PET quantification of tau pathology with 11C-PBB3 (2-((1E,3E)-4-(6-(11C-methylamino)pyridin-3-yl)buta-1,3-dienyl) benzo[d]thiazol-6-ol), considering its radiometabolite entering the brain. Methods: Seven Alzheimer disease patients and 7 healthy subjects underwent dynamic 11C-PBB3 PET scanning. Arterial blood was sampled to obtain the parent and metabolite input functions. Quantification of 11C-PBB3 binding was performed using dual-input models that take the brain metabolite activity into consideration, traditional single-input models without such considerations, and the reference tissue model (MRTMO) and standardized uptake value ratio (SUVR). The cerebellar cortex was used as the reference tissue for all methods. Results: The dual-input graphical models estimated binding parameter (BPND*) stably (∼0.36 in high-binding regions). The MRTMO BPND* matched the corresponding BPND* by the dual-input graphical model (r2 = 1.00). SUVR minus 1 correlated well with MRTMO BPND* (r2 > 0.97). However, BPND by the single-input models did not correlate with BPND* by the dual-input graphical model (r2 = 0.04). Conclusion: The dual-input graphical model BPND* is consistent with the reference tissue BPND* and SUVR-1, suggesting that these parameters can accurately quantify binding of 11C-PBB3 despite the entry of its radiometabolites into the brain.

Quantification of Dopamine Transporter in Human Brain Using PET with 18F-FE-PE2I

18F-(E)-N-(3-iodoprop-2E-enyl)-2β-carbofluoroethoxy-3β-(4-methylphenyl)nortropane (18F-FE-PE2I) is a new PET radioligand with a high affinity and selectivity for the dopamine transporter (DAT). In nonhuman primates, 18F-FE-PE2I showed faster kinetics and less production of radiometabolites that could potentially permeate the blood–brain barrier than did 11C-PE2I. The aims of this study were to examine the quantification of DAT using 18F-FE-PE2I and to assess the effect of radiometabolites of 18F-FE-PE2I on the quantification in healthy humans. Methods: A 90-min dynamic PET scan was obtained for 10 healthy men after intravenous injection of 18F-FE-PE2I. Kinetic compartment model analysis with a metabolite-corrected arterial input function was performed. The effect of radiometabolites on the quantification was evaluated by time-stability analyses. The simplified reference tissue model (SRTM) method with the cerebellum as a reference region was evaluated as a noninvasive method of quantification. Results: After the injection of 18F-FE-PE2I, the whole-brain radioactivity showed a high peak (∼3–5 standardized uptake value) and fast washout. The radioactive uptake of 18F-FE-PE2I in the brain was according to the relative density of the DAT (striatum > midbrain > thalamus). The cerebellum showed the lowest uptake. Tissue time–activity curves were well described by the 2-tissue-compartment model (TCM), as compared with the 1-TCM, for all subjects in all regions. Time stability analysis showed stable estimation of total distribution volume with 60-min or longer scan durations, indicating the small effect of radiometabolites. Binding potentials in the striatum and midbrain were well estimated by the SRTM method, with modest intersubject variability. Although the SRTM method yielded a slight underestimation and overestimation in regions with high and low DAT densities, respectively, binding potentials by the SRTM method were well correlated to the estimates by the indirect kinetic method with 2-TCM. Conclusion: 18F-FE-PE2I is a promising PET radioligand for quantifying DAT. The binding potentials could be reliably estimated in both the striatum and midbrain using both the indirect kinetic and SRTM methods with a scan duration of 60 min. Although radiometabolites of 18F-FE-PE2I in plasma possibly introduced some effects on the radioactivity in the brain, the effects on estimated binding potential were likely to be small.

Quantitative Analysis of Norepinephrine Transporter in the Human Brain Using PET with (S,S)-18F-FMeNER-D2

(S,S)-18F-FMeNER-D2 was recently developed as a radioligand for the measurement of norepinephrine transporter imaging with PET. In this study, a norepinephrine transporter was visualized in the human brain using this radioligand with PET and quantified by several methods. Methods: PET scans were performed on 10 healthy men after intravenous injection of (S,S)-18F-FMeNER-D2. Binding potential relative to nondisplaceable binding (BPND) was quantified by the indirect kinetic, simplified reference-tissue model (SRTM), multilinear reference-tissue model (MRTM), and ratio methods. The indirect kinetic method was used as the gold standard and was compared with the SRTM method with scan times of 240 and 180 min, the MRTM method with a scan time of 240 min, and the ratio method with a time integration interval of 120–180 min. The caudate was used as reference brain region. Results: Regional radioactivity was highest in the thalamus and lowest in the caudate during PET scanning. BPND values by the indirect kinetic method were 0.54 ± 0.19 and 0.35 ± 0.25 in the thalamus and locus coeruleus, respectively. BPND values found by the SRTM, MRTM, and ratio methods agreed with the values demonstrated by the indirect kinetic method (r = 0.81–0.92). Conclusion: The regional distribution of (S,S)-18F-FMeNER-D2 in our study agreed with that demonstrated by previous PET and postmortem studies of norepinephrine transporter in the human brain. The ratio method with a time integration interval of 120–180 min will be useful for clinical research of psychiatric disorders for estimation of norepinephrine transporter occupancy by antidepressants without requiring arterial blood sampling and dynamic PET.

Microvascular sprouting, extension, and creation of new capillary connections with adaptation of the neighboring astrocytes in adult mouse cortex under chronic hypoxia

The present study aimed to determine the spatiotemporal dynamics of microvascular and astrocytic adaptation during hypoxia-induced cerebral angiogenesis. Adult C57BL/6J and Tie2-green fluorescent protein (GFP) mice with vascular endothelial cells expressing GFP were exposed to normobaric hypoxia for 3 weeks, whereas the three-dimensional microvessels and astrocytes were imaged repeatedly using two-photon microscopy. After 7 to14 days of hypoxia, a vessel sprout appeared from the capillaries with a bump-like head shape (mean diameter 14 μm), and stagnant blood cells were seen inside the sprout. However, no detectable changes in the astrocyte morphology were observed for this early phase of the hypoxia adaptation. More than 50% of the sprouts emerged from capillaries 60 μm away from the center penetrating arteries, which indicates that the capillary distant from the penetrating arteries is a favored site for sprouting. After 14 to 21 days of hypoxia, the sprouting vessels created a new connection with an existing capillary. In this phase, the shape of the new vessel and its blood flow were normalized, and the outside of the vessels were wrapped with numerous processes from the neighboring astrocytes. The findings indicate that hypoxia-induced cerebral angiogenesis provokes the adaptation of neighboring astrocytes, which may stabilize the blood–brain barrier in immature vessels.

Quantitative Analysis of Peripheral Benzodiazepine Receptor in the Human Brain Using PET with 11C-AC-5216

Peripheral benzodiazepine receptor (PBR) is upregulated in activated glial cells and is therefore a useful biomarker for inflammation in the brain and neurodegenerative disorders. We developed a new PET radioligand, 11C-AC-N-benzyl-N-ethyl-2-(7-methyl-8-oxo-2-pheyl-7,8-dihydro-9H-purin-9-yl)acetamide (11C-AC-5216), that allows the imaging and quantification of PBRs in monkey and mouse brains. The aim of this study was to evaluate a quantification method of 11C-AC-5216 binding in the human brain. Methods: A 90-min dynamic PET scan was obtained for each of 12 healthy men after an intravenous injection of 11C-AC-5216. Regions of interest were drawn on several brain regions. Binding potential, compared with nondisplaceable uptake (BPND), was calculated by a nonlinear least-squares fitting (NLS) method with the 2-tissue-compartment model, and total volume of distribution (VT) was estimated by NLS and graphical analysis methods. Results: BPND was highest in the thalamus (4.6 ± 1.0) and lowest in the striatum (3.5 ± 0.7). VT obtained by NLS or graphical analysis showed regional distribution similar to BPND. However, there was no correlation between BPND and VT because of the interindividual variation of K1/k2. BPND obtained with data from a scan time of 60 min was in good agreement with that from a scan time of 90 min (r = 0.87). Conclusion: Regional distribution of 11C-AC-5216 was in good agreement with previous PET studies of PBRs in the human brain. BPND is more appropriate for estimating 11C-AC-5216 binding than is VT because of the interindividual variation of K1/k2. 11C-AC-5216 is a promising PET ligand for quantifying PBR in the human brain.

Hemodynamics evoked by microelectrical direct stimulation in rat somatosensory cortex

The aim of this study was to estimate the timing (latency) of the increase in red blood cell (RBC) velocity and RBC concentration, and the magnitude of response in local cerebral blood flow (LCBF) for neuronal activation. We measured LCBF change during activation of the somatosensory cortex by direct microelectrical stimulation. Electrical stimuli of 5, 10 and 50 Hz of 1 ms pulse with 10-15 microA, were given for 5 s. LCBF, RBC velocity and RBC concentration were monitored by laser-Doppler flowmetry (LDF) in alpha-chloralose anesthetized rats (n = 7). LCBF, RBC velocity and RBC concentration increased nearly proportionally to stimulus frequency, i.e. neuronal activity. LCBF rose approximately 0.5 s after the onset of stimulation, and there was no significant time lag of the latencies among LCBF, RBC velocity and RBC concentration at the same stimulus frequency. We interpret these results to mean that the onset of LCBF increase on cortical activation is reflected by a rapid change in arteriole (resistance vessel) dilation and capillary volume. The data also elucidate the linear relationship between LCBF increase and cortical activity.

Quantitative PET Analysis of the Dopamine D2 Receptor Agonist Radioligand 11C-(R)-2-CH3O-N-n-Propylnorapomorphine in the Human Brain

It has been demonstrated in vitro that the dopamine D2 receptor has 2 interconvertible affinity states for endogenous dopamine, referred to as the high- and the low-affinity states. 11C-(R)-2-CH3O-N-n-propylnorapomorphine (11C-MNPA) is a new agonist radioligand for in vivo imaging of the high-affinity state of dopamine D2 receptors using PET. In the present study, the kinetics of 11C-MNPA were examined for the first time, to our knowledge, in the human brain and analyzed using quantitative approaches with or without an arterial input function. Methods: A 90-min dynamic PET scan was obtained for 10 healthy men after an intravenous injection of 11C-MNPA. The binding potential (BPND) was calculated using the indirect kinetic method, a kinetic compartment analysis with a metabolite-corrected arterial input function. BPND was also calculated by the simplified reference tissue model (SRTM) and transient equilibrium methods, both with the cerebellum as the reference brain region. The results of the quantitative methods were compared in a cross-validation approach. Results: The highest regional radioactivity was observed in the putamen. BPND values obtained by kinetic analysis were 0.82 ± 0.09, 0.59 ± 0.11, and 0.28 ± 0.06, respectively, in the putamen, caudate, and thalamus. BPND values obtained by the SRTM and transient equilibrium methods were in good agreement with those obtained by the indirect kinetic method (r = 0.98 and r = 0.93, respectively). For all quantification methods, the BPND values based on data acquired from 0 to 60 min were in good agreement with those based on data acquired from 0 to 90 min (r = 0.90–0.99). Conclusion: The regional distribution of 11C-MNPA binding was in good agreement with previous PET studies of dopamine D2 receptors in the human brain using antagonist radioligands. The results support routine use of the SRTM and transient equilibrium methods, that is, methods that do not require an arterial input function and need a scan time of only about 60 min. 11C-MNPA should thus be useful for clinical research on the pathophysiology of neuropsychiatric disorders and estimation of dopamine D2 receptor occupancy by dopaminergic drugs.

Changes in cortical microvasculature during misery perfusion measured by two-photon laser scanning microscopy

This study aimed to examine the cortical microvessel diameter response to hypercapnia in misery perfusion using two-photon laser scanning microscopy (TPLSM). We evaluated whether the vascular response to hypercapnia could represent the cerebrovascular reserve. Cerebral blood flow (CBF) during normocapnia and hypercapnia was measured by laser-Doppler flowmetry through cranial windows in awake C57/BL6 mice before and at 1,7, 14, and 28 days after unilateral common carotid artery occlusion (UCCAO). Diameters of the cortical microvessels during normocapnia and hypercapnia were also measured by TPLSM. Cerebral blood flow and the vascular response to hypercapnia were decreased after UCCAO. Before UCCAO, vasodilation during hypercapnia was found primarily in arterioles (22.9% ± 3.5%). At 14 days after UCCAO, arterioles, capillaries, and venules were autoregulatorily dilated by 79.5% ± 19.7%, 57.2% ±32.3%, and 32.0% ± 10.8%, respectively. At the same time, the diameter response to hypercapnia in arterioles was significantly decreased to 1.9% ± 1.5%. A significant negative correlation was observed between autoregulatory vasodilation and the diameter response to hypercapnia in arterioles. Our findings indicate that arterioles play main roles in both autoregulatory vasodilation and hypercapnic vasodilation, and that the vascular response to hypercapnia can be used to estimate the cerebrovascular reserve.

Drugs interacting with organic anion transporter-1 affect uptake of Tc-99m-mercaptoacetyl-triglycine (MAG3) in the human kidney: Therapeutic drug interaction in Tc-99m-MAG3 diagnosis of renal function and possible application of Tc-99m-MAG3 for drug development

INTRODUCTION Renal uptake of Tc-99m-MG3 involves organic anion transporter (OAT). Treatment with drugs showing OAT affinity might interfere with renal uptake of Tc-99m-MAG3, leading to misinterpretation in Tc-99m-MAG3. This study was conducted to discuss a possible drug interference with Tc-99m-MAG3 diagnosis on OAT sites. METHODS Renal uptake and plasma clearance of Tc-99m-MAG3 were analyzed in healthy volunteers under control and OAT1 and OAT3 related drug treatment conditions. An in vitro uptake study using OAT1 or OAT3 expressing cells was also conducted. RESULTS Both PAH and probenecid treatment induced delays in Tc-99m-MAG3 clearance from blood, and reductions in the renal uptake clearance. As a result, the normalized effective renal plasma flow estimated from Tc-99m-MAG3 clearance was significantly underestimated, whereas the glomerular filtration rate estimated from plasma creatinine levels was unchanged. The transport activity of Tc-99m-MAG3 was higher in OAT1-expressing cells than in OAT3-expressing cells. CONCLUSION Drugs with OAT1 affinity affect the renal uptake of Tc-99m-MAG3 and blood clearance. This might cause misinterpretation of functional diagnosis of the kidney using Tc-99m-MAG3.

Assessment of radioligands for PET imaging of cyclooxygenase-2 in an ischemic neuronal injury model

Cyclooxygenase-2 (COX-2) plays crucial roles in progressive neuronal death in ischemic brain injury. In the present study, we evaluated two radiolabeled COX-2 selective inhibitors, [11C]celecoxib and [11C]rofecoxib, as positron emission tomography (PET) tracers for COX-2 imaging in normal and ischemic mouse brains. We also took advantage of our newly-generated antibody highly selective for mouse COX-2 to prove accumulation of the radioligands in regions enriched with COX-2. In vitro autoradiography demonstrated specific binding of high-concentration [11C]rofecoxib but not [11C]celecoxib to the cerebellum and brain stem of normal brains wherein COX-2 immunoreactivity in neurons was most abundantly observed. Meanwhile, both of these radioligands failed to detect COX-2 expression in PET assays despite their excellent brain permeability. Hypoperfusion-induced ischemia caused marked necrotic neuron death accompanied by gliosis and enhancement of neuronal COX-2 immunoreactivity in the hippocampus. Correspondingly, in vitro autoradiographic binding of [11C]rofecoxib was increased in the injured hippocampus compared to the uninjured contralateral region, but failed in living brains of ischemia model likewise. Our work provides the rationale for monitoring COX-2 as a biomarker reflecting ischemic brain injuries and demonstrates that [11C]rofecoxib, not [11C]celecoxib, is useful for in vitro assays of COX-2, but its affinity would be insufficient for in vivo PET visualization.