Neuropathologic heterogeneity is often present within Alzheimer’s disease (AD). We sought to determine if amyloid imaging measures of AD are affected by concurrent pathologies. Thirty-eight clinicopathologically-defined AD and 17 non-demented cases (ND) with quantitative florbetapir F-18 (18F-AV-45) PET imaging during life and histological β-amyloid quantification and neuropathologic examination were assessed. AD cases were divided on the basis of concurrent pathologies, including those with Lewy bodies (N=21), white matter rarefaction (N=27), severe cerebral amyloid angiopathy (N=11), argyrophilic grains (N=5) and TDP-43 inclusions (N=18). Many cases exhibited more than one type of concurrent pathology. The ratio of cortical to cerebellar amyloid imaging signal (SUVr) and immunohistochemical β-amyloid load were analyzed in six cortical regions of interest. All AD subgroups had strong and significant correlations between SUVr and histological β-amyloid measures (p values <0.001). All AD subgroups had significantly greater amyloid measures compared to ND, and mean amyloid measures did not significantly differ between AD subgroups. When comparing AD cases with and without each pathology, AD cases with Lewy bodies had significantly decreased SUVr measures compared to AD cases without (p = 0.002); there were no other paired comparison differences. These findings indicate florbetapir-PET imaging is not confounded by neuropathological heterogeneity within AD.
argyrophilic grains; autopsy; cerebral amyloid angiopathy; Lewy bodies; plaques; TDP-43; vascular dementia; white matter; leuko-araiosis
Biomarkers based on the underlying pathology of Alzheimer’s disease (AD) and Dementia with Lewy Bodies (DLB) have the potential to improve diagnosis and understanding of the substrate for cognitive impairment in these disorders. The objective of this study was to compare the patterns of amyloid and dopamine PET imaging in patients with AD, DLB and Parkinson’s disease (PD) using the amyloid imaging agent florbetapir F 18 and 18F-AV-133 (florbenazine), a marker for vesicular monamine type 2 transporters (VMAT2).
Patients with DLB and AD, Parkinson’s disease (PD) and healthy controls (HC) were recruited for this study. On separate days, subjects received intravenous injections of florbetapir, and florbenazine. Amyloid burden and VMAT2 density were assessed quantitatively and by binary clinical interpretation. Imaging results for both tracers were compared across the four individual diagnostic groups and for combined groups based on underlying pathology (AD/DLB vs. PD/HC for amyloid burden and PD/DLB vs. AD/HC for VMAT binding) and correlated with measures of cognition and parkinsonism.
11 DLB, 10 AD, 5 PD, and 5 controls participated in the study. Amyloid binding was significantly higher in the combined AD/DLB patient group (n = 21) compared to the PD/HC groups (n = 10, mean SUVr: 1.42 vs. 1.07; p = 0.0006). VMAT2 density was significantly lower in the PD/DLB group (n = 16) compared to the AD/ HC group (n = 15; 1.83 vs. 2.97; p < 0.0001). Within the DLB group, there was a significant correlation between cognitive performance and striatal florbenazine binding (r = 0.73; p = 0.011).
The results of this study show significant differences in both florbetapir and florbenazine imaging that are consistent with expected pathology. In addition, VMAT density correlated significantly with cognitive impairment in DLB patients (ClinicalTrials.gov identifier: NCT00857506, registered March 5, 2009).
PET imaging; Alzheimer’s disease; Parkinson’s disease; Biomarkers
The objective of this study was to evaluate the relationship of amyloid burden, as assessed by florbetapir F 18 (18F-AV-45) amyloid PET, and cognition in healthy older control subjects (HC). Seventy-eight HC subjects were assessed with a brief cognitive test battery and PET imaging with florbetapir F 18. A standard uptake value ratio (SUVr) was computed for mean data from six cortical regions using a whole cerebellum reference region. Scans were also visually rated as amyloid positive (Aβ+) or amyloid negative (Aβ−) by three readers. Higher SUVr correlated with lower immediate memory (r=−0.33; p=0.003) and delayed recall scores (r=−0.25; p=0.027). Performance on immediate recall was also lower in the visually rated Aβ+ compared to Aβ− HC (p=0.04), with a similar trend observed in delayed recall (p=0.06). These findings support the hypothesis that higher amyloid burden is associated with lower memory performance among clinically normal older subjects. Longitudinal follow-up is ongoing to determine whether florbetapir F 18 may also predict subsequent cognitive decline.
Florbetapir is one of several 18F-labeled amyloid plaque imaging tracers for positron emission tomography (PET). As the bio-distribution and radiation dose of PET tracers in human research are important for estimating the relative risks and benefits, a study was conducted to obtain this information on florbetapir.
Nine cognitively normal subjects (six females and three males, age 58 ± 10 years, weight 81 ± 17 kg) received an intravenous bolus injection of 395 ± 27.9 MBq of florbetapir, and whole-body emission scans were performed over approximately 6 h. Computed tomography scans were acquired for attenuation correction. Volumes of interest (VOIs) for source organs including the brain, liver, lung, heart wall, and vertebrae were defined on the PET images. The VOIs of the gallbladder, urinary bladder, and large and small intestines were also defined. Using reference man organ volumes (ICRP 30), total activity was calculated per organ for each time point. The resultant time-activity curves (TACs) were fitted with constrained exponentials. Kinetic data were entered into OLINDA/EXM software to calculate dose estimates; the dynamic urinary bladder and ICRP 30 GI tract models were employed. The effective dose (ED) for each subject was estimated from the acquired data using the adult model.
The mean ED determined for nine healthy volunteers was 18.60 ± 4.26 μSv/MBq or 6.88 mSv for a 370-MBq dose. The organs that received the highest radiation absorbed doses were the gallbladder, upper large intestine, small intestine, liver, and urinary bladder at 143.0 ± 80.20, 74.50 ± 34.20, 65.50 ± 29.60, 64.40 ± 22.10, and 27.10 ± 11.70 μSv/MBq, respectively.
The ED for florbetapir has been calculated for nine healthy volunteers. At a dose of 370 MBq florbetapir, the total average ED is approximately 6.88 mSv.
Florbetapir F 18 PET can image amyloid-β (Aβ) aggregates in the brains of living subjects. We prospectively evaluated the prognostic utility of detecting Aβ pathology using florbetapir PET in subjects at risk for progressive cognitive decline.
A total of 151 subjects who previously participated in a multicenter florbetapir PET imaging study were recruited for longitudinal assessment. Subjects included 51 with recently diagnosed mild cognitive impairment (MCI), 69 cognitively normal controls (CN), and 31 with clinically diagnosed Alzheimer disease dementia (AD). PET images were visually scored as positive (Aβ+) or negative (Aβ−) for pathologic levels of β-amyloid aggregation, blind to diagnostic classification. Cerebral to cerebellar standardized uptake value ratios (SUVr) were determined from the baseline PET images. Subjects were followed for 18 months to evaluate changes in cognition and diagnostic status. Analysis of covariance and correlation analyses were conducted to evaluate the association between baseline PET amyloid status and subsequent cognitive decline.
In both MCI and CN, baseline Aβ+ scans were associated with greater clinical worsening on the Alzheimer's Disease Assessment Scale–Cognitive subscale (ADAS-Cog (p < 0.01) and Clinical Dementia Rating–sum of boxes (CDR-SB) (p < 0.02). In MCI Aβ+ scans were also associated with greater decline in memory, Digit Symbol Substitution (DSS), and Mini-Mental State Examination (MMSE) (p < 0.05). In MCI, higher baseline SUVr similarly correlated with greater subsequent decline on the ADAS-Cog (p < 0.01), CDR-SB (p < 0.03), a memory measure, DSS, and MMSE (p < 0.05). Aβ+ MCI tended to convert to AD dementia at a higher rate than Aβ− subjects (p < 0.10).
Florbetapir PET may help identify individuals at increased risk for progressive cognitive decline.
The ability to non-invasively measure endogenous pancreatic β-cell mass (BCM) would accelerate research on the pathophysiology of diabetes and revolutionize the preclinical development of new treatments, the clinical assessment of therapeutic efficacy, and the early diagnosis and subsequent monitoring of disease progression. The vesicular monoamine transporter type 2 (VMAT2) is co-expressed with insulin in β-cells and represents a promising target for BCM imaging.
We evaluated the VMAT2 radiotracer 18F-fluoropropyl-dihydrotetrabenazine ([18F]FP-(+)-DTBZ, also known as [18F]AV-133) for quantitative positron emission tomography (PET) imaging of BCM in healthy control subjects and patients with type 1 diabetes mellitus (T1DM). Standardized uptake value (SUV) was calculated as the net tracer uptake in pancreas normalized by injected dose and body weight. Total volume of distribution (VT), the equilibrium ratio of tracer concentration in tissue relative to plasma, was estimated by kinetic modeling with arterial input functions. Binding potential (BPND), the steady-state ratio of specific binding to non-displaceable uptake, was calculated using the renal cortex as a reference tissue devoid of specific VMAT2 binding.
Mean pancreatic SUV, VT, and BPND were reduced by 38%, 20% and 40%, respectively, in T1DM. The radiotracer binding parameters correlated with insulin secretion capacity as determined by arginine-stimulus tests. Group differences and correlations with β-cell function were enhanced for total pancreas binding parameters that accounted for tracer binding density as well as organ volume.
These findings demonstrate that quantitative evaluation of islet β-cell density and aggregate BCM can be performed clinically with [18F]FP-(+)-DTBZ PET.
Diabetes; pancreas; beta cell mass; PET
Florbetapir F 18 (18F-AV-45) is a positron emission tomography (PET) imaging ligand for the detection of amyloid aggregation associated with Alzheimer’s disease. Earlier data showed that florbetapir F 18 binds with high affinity to β-amyloid plaques in human brain homogenates (Kd = 3.7 nM) and has favorable imaging pharmacokinetic properties, including rapid brain penetration and washout. The present study used human autopsy brain tissue to evaluate the correlation between in vitro florbetapir F 18 binding and β-amyloid density measured by established neuropathological methods.
The localization and density of florbetapir F 18 binding in frozen and formalin-fixed paraffin-embedded sections of postmortem brain tissue from 40 subjects with a varying degree of neurodegenerative pathology was assessed by standard florbetapir F 18 autoradiography and correlated with the localization and density of β-amyloid identified by silver staining, thioflavin S staining, and immunohistochemistry.
There were strong quantitative correlations between florbetapir F 18 tissue binding and both β-amyloid plaques identified by light microscopy (sliver staining and thioflavin S fluorescence) and by immunohistochemical measurements of β-amyloid using three antibodies recognizing different epitopes of the β-amyloid peptide (Aβ). Florbetapir F 18 did not bind to neurofibrillary tangles.
Florbetapir F 18 selectively binds β-amyloid in human brain tissue. The binding intensity was quantitatively correlated with the density of β-amyloid plaques identified by standard neuropathological techniques and correlated with the density of Aβ measured by immunohistochemistry. Since β-amyloid plaques are a defining neuropathological feature for Alzheimer’s disease, these results support the use of florbetapir F 18 as an amyloid PET ligand to identify the presence of AD pathology in patients with signs and symptoms of progressive late-life cognitive impairment.
PET imaging; Alzheimer’s disease; β-amyloid plaque; autoradiography; β-amyloid; amyloid PET imaging; florbetapir F 18; 18F-AV-45; postmortem
Beta-amyloid plaques (Aβ plaques) in the brain are associated with cerebral amyloid angiopathy (CAA). Imaging agents that could target the Aβ plaques in the living human brain would be potentially valuable as biomarkers in patients with CAA. A new series of 18F styrylpyridine derivatives with high molecular weights for selectively targeting Aβ plaques in the blood vessels of the brain, but excluded from the brain parenchyma is reported. The styrylpyridine derivatives, 8a–c, display high binding affinities and specificity to Aβ plaques (Ki = 2.87 nM, 3.24 and 7.71 nM, respectively). In vitro autoradiography of [18F]8a shows labeling of β-amyloid plaques associated with blood vessel walls in human brain sections of subjects with CAA, and also in the tissue of AD brain sections. The results suggest that [18F]8a may be a useful PET imaging agent for selectively detecting Aβ plaques associated with cerebral vessels in the living human brain.
PET imaging; Alzheimer’s disease; cerebral blood vessels; β–amyloid plaque; cerebral amyloid angiopathy; autoradiography and in vivo biodistribution
Down syndrome appears to be associated with a virtually certain risk of fibrillar amyloid-β (Aβ) pathology by the age of 40 and a very high risk of dementia at older ages. The positron emission tomography (PET) ligand florbetapir F18 has been shown to characterize fibrillar Aβ in the living human brain and to provide a close correlation with subsequent Aβ neuropathology in individuals proximate to and after the end of life. The extent to which the most frequently used PET ligands can be used to detect fibrillar Aβ in patients with Down syndrome remains to be determined.
To characterize PET estimates of fibrillar Aβ burden in a Down syndrome patient very close to the end of life and to compare them with neuropathologic assessment made after his death.
With the family’s informed consent, florbetapir PET was used to study a 55-year-old Down syndrome patient with Alzheimer disease near the end of life; his brain was donated for neuropathologic assessment when he died 14 days later. Visual ratings of cerebral florbetapir uptake were performed by trained readers who were masked to the patient’s diagnosis as part of a larger study, and an automated algorithm was used to characterize regional-to-cerebellar standard uptake value ratios in 6 cerebral regions of interest. Neuropathologic assessments were performed masked to the patient’s diagnosis or PET measurements.
Visual ratings and automated analyses of the PET image revealed a heavy fibrillar Aβ burden in cortical, striatal, and thalamic regions, similar to that reported for patients with late-onset Alzheimer disease. This matched neuropathologic findings of frequent neuritic and diffuse plaques, as well as frequent amyloid angiopathy, except for neuropathologically demonstrated frequent cerebellar diffuse plaques and amyloid angiopathy that were not detected by the PET scan.
Florbetapir PET can be used to detect increased cerebral-to-cerebellar fibrillar Aβ burden in a Down syndrome patient with Alzheimer disease, even in the presence of frequent amyloid angiopathy and diffuse plaques in the cerebellum. Additional studies are needed to determine the extent to which PET could be used to detect and to track fibrillar Aβ and to evaluate investigational Aβ-modifying treatments in the presymptomatic and symptomatic stages of Alzheimer disease.
An [18F] labeled PET amyloid (Aβ) imaging agent could facilitate clinical evaluation of late-life cognitive impairment by providing an objective measure for Alzheimer’s disease (AD) pathology. Here we present the results of the first clinical trial with [18F]AV-45 (Florbetapir F 18).
An open-label, multicenter, brain imaging, metabolism and safety study of [18F]AV-45 was performed on 16 patients with Alzheimer’s disease (AD: MMSE 19.3 +/− 3.1; Age 75.8 +/− 9.2) and 16 cognitively healthy controls (HC: MMSE 29.8 +/− 0.45; Age 72.5 +/− 11.6 ). Dynamic PET imaging was performed over a period of approximately 90 minutes following 10 mCi injection of the tracer. Standard uptake values (SUV) and cortical to cerebellum SUV ratios (SUVR) were calculated. A simplified reference tissue method was used to generate distribution volume ratio (DVR) parametric maps in a subset of subjects
Valid PET imaging data were available for 11 AD and 15 HC subjects [18F]AV-45 accumulated in cortical regions expected to be high in amyloid deposition (e.g., precuneus, frontal and temporal cortex) of AD patients; minimal accumulation of tracer was seen in cortical regions of HC subjects. The cortical to cerebellar SUVR values in AD patients showed continual substantial increases through 30 minutes post-administration, reaching a plateau within 50 minutes. The 10 minute period from 50–60 minutes post administration was taken as a representative sample for further analysis. The cortical average SUVR for this period was 1.67 +/− 0.175 for patients with AD vs. 1.25 +/− 0.177 for HC subjects. Spatially normalized DVRs generated from PET dynamic scans were highly correlated with SUVR (r= 0.58–0.88, p<0.005) and were significantly greater for AD patients than for HC subjects in cortical regions, but not in subcortical white matter or cerebellar regions.
There were no clinically significant changes in vital signs, ECG or laboratory values.
[18F]AV-45 was well tolerated and PET imaging showed significant discrimination between AD patients and HC subjects using either a parametric reference region method (DVR) or a simplified SUVR calculated from 10 minutes of scanning 50–60 minutes after [18F]AV-45 administration.
Amyloid; PET; Alzheimer’s Disease; Dementia; Biomarkers; Aging; F-18
This is a progress report of the Alzheimer's Disease Neuroimaging Initiative (ADNI) PET Core.
The Core has supervised the acquisition, quality control, and analysis of longitudinal [18F]fluorodeoxyglucose PET (FDG-PET) data in approximately half of the ADNI cohort. In an “add on” study, approximately 100 subjects also underwent scanning with [11C]PIB-PET for amyloid imaging. The Core developed quality control procedures and standardized image acquisition by developing an imaging protocol that has been widely adopted in academic and pharmaceutical industry studies. Data processing provides users with scans that have identical orientation and resolution characteristics despite acquisition on multiple scanner models. The Core labs have used a number of different approaches to characterize differences between subject groups (AD, MCI, controls), to examine longitudinal change over time in glucose metabolism and amyloid deposition, and to assess the use of FDG-PET as a potential outcome measure in clinical trials.
ADNI data indicate that FDG-PET increases statistical power over traditional cognitive measures, might aid subject selection, and could substantially reduce the sample size in a clinical trial. PIB-PET data showed expected group differences, and identified subjects with significant annual increases in amyloid load across the subject groups. The next activities of the PET core in ADNI will entail developing standardized protocols for amyloid imaging using the [18F]-labeled amyloid imaging agent AV45, which can be delivered to virtually all ADNI sites.
ADNI has demonstrated the feasibility and utility of multicenter PET studies and is helping to clarify the role of biomarkers in the study of aging and dementia.
PET; fluorodeoxyglucose; amyloid imaging; biomarkers
β-amyloid plaques (Aβ plaques) in the brain, containing predominantly fibrillary Aβ peptide aggregates, represent a defining pathologic feature of Alzheimer disease (AD). Imaging agents targeting the Aβ plaques in the living human brain are potentially valuable as biomarkers of pathogenesis processes in AD. (E)-4-(2-(6-(2-(2-(2-18F-fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N-methyl benzenamine (18F-AV-45) is such as an agent currently in phase III clinical studies for PET of Aβ plaques in the brain.
In vitro binding of 18F-AV-45 to Aβ plaques in the postmortem AD brain tissue was evaluated by in vitro binding assay and autoradiography. In vivo biodistribution of 18F-AV-45 in mice and ex vivo autoradiography of AD transgenic mice (APPswe/PSEN1) with Aβ aggregates in the brain were performed. Small-animal PET of a monkey brain after an intravenous injection of 18F-AV-45 was evaluated.
18F-AV-45 displayed a high binding affinity and specificity to Aβ plaques (Kd, 3.72 ± 0.30 nM). In vitro autoradiography of postmortem human brain sections showed substantial plaque labeling in AD brains and not in the control brains. Initial high brain uptake and rapid washout from the brain of healthy mice and monkey were observed. Metabolites produced in the blood of healthy mice after an intravenous injection were identified. 18F-AV-45 displayed excellent binding affinity to Aβ plaques in the AD brain by ex vivo autoradiography in transgenic AD model mice. The results lend support that 18F-AV-45 may be a useful PET agent for detecting Aβ plaques in the living human brain.
PET imaging; Alzheimer disease; β-amyloid plaque; autoradiography; biodistribution
Development of imaging agents for pancreatic beta cell mass may provide tools for studying insulin-secreting beta cells and their relationship with diabetes mellitus. In this paper a new imaging agent, [18F](+)-2-oxiranyl-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline [18F](+)4, which displays properties targeting vesicular monoamine transporter 2 (VMAT2) binding sites of beta cells in the pancreas, was evaluated as a PET (positron emission tomography) agent for estimating beta cell mass in vivo. The hydrolyzable epoxide group of (+)4 may provide a mechanism for shifting biodistribution from liver to kidney thus, reducing the background signal.
Both 18F and 19F labeled (+) and (−) isomers of 4 were synthesized and evaluated. Organ distribution was carried out in normal rats. Uptake of [18F](+)4 in pancreas of normal rats was measured and correlated with blocking studies using competing drugs, (+)dihydrotetrabenazine, (+)-DTBZ or 9-fluoropropyl-(+)dihydro tetrabenazine (FP-(+)-DTBZ, (+)2).
In vitro binding study of VMAT2 using rat brain striatum showed a Ki value of 0.08 and 0.15 nM for the (+)4 and (±)4, respectively. The in vivo biodistribution of [18F](+)4 in rats showed the highest uptake in the pancreas (2.68 %ID/g at 60 min post-injection). In vivo competition experiments with cold FP-(+)-DTBZ, (+)2, (3.5 mg/kg, 5 min iv pretreatment) led to a significant reduction of pancreas uptake (85 % blockade at 60 min). The inactive isomer [18F](−)4 showed significantly lower pancreas uptake (0.22 %ID/g at 30 min post-injection). Animal PET imaging studies of [18F](+)4 in normal rats demonstrated an avid pancreatic uptake in rats.
The preliminary results suggest that the epoxide, [18F](+)4, is highly selective in binding to VMAT2 and it has an excellent uptake in the pancreas of rats. The liver uptake was significantly reduced through the use of the epoxide group. Therefore, it may be potentially useful for imaging beta cell mass in the pancreas.
Beta cell mass; vesicular monoamine transporter 2; diabetes; tetrabenazine and PET
Two new phenylacetylene derivatives, 5-((4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)phenyl)ethynyl)indoline 8 and 5-((4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)phenyl)ethynyl)-1H-indole 14, targeting β-amyloid (Aβ) plaques have been prepared. In vitro binding carried out in tissue homogenates prepared from postmortem AD brains with [125I]IMPY (6-iodo-2-(4’-dimethylamino-)phenylimidazo[1,2-a]pyridine) as the radioligand indicated good binding affinities (Ki = 4.0 and 1.5 nM for 8 and 14, respectively). Brain penetration of the corresponding radiofluorinated ligands, evaluated in the normal mice, showed good initial brain penetration (4.50 and 2.43% ID/g (injected dose/gram) for [18F]8 and [18F]14 at 2 min after injection) with moderate to low washout rates from the brain (1.71% ID/g at 2 h and 2.10% ID/g at 3 h, respectively). Autoradiography and homogenate binding studies demonstrated the high specific binding of [18F]14 to the Aβ plaques; however, [18F]8 showed low specific binding. These preliminary results identified that indolylphenylacetylene, 14, may be a good lead for further structural modification to develop a useful Aβ plaque imaging agent.
The amyloid-β peptide (Aβ) is produced at several sites within cultured human NT2N neurons with Aβ1-42 specifically generated in the endoplasmic reticulum/intermediate compartment. Since Aβ is found as insoluble deposits in senile plaques of the AD brain, and the Aβ peptide can polymerize into insoluble fibrils in vitro, we examined the possibility that Aβ1-40, and particularly the more highly amyloidogenic Aβ1-42, accumulate in an insoluble pool within NT2N neurons. Remarkably, we found that formic acid extraction of the NT2N cells solubilized a pool of previously undetectable Aβ that accounted for over half of the total intracellular Aβ. Aβ1-42 was more abundant than Aβ1-40 in this pool, and most of the insoluble Aβ1-42 was generated in the endoplasmic reticulum/intermediate compartment pathway. High levels of insoluble Aβ were also detected in several nonneuronal cell lines engineered to overexpress the amyloid-β precursor protein. This insoluble intracellular pool of Aβ was exceptionally stable, and accumulated in NT2N neurons in a time-dependent manner, increasing 12-fold over a 7-wk period in culture. These novel findings suggest that Aβ amyloidogenesis may be initiated within living neurons rather than in the extracellular space. Thus, the data presented here require a reexamination of the prevailing view about the pathogenesis of Aβ deposition in the AD brain.